fix(subagent): wait on idle stall pings instead of inferring

## Summary

An empty stall or compaction continuation with recent activity
waits instead of starting a model turn. Compact resume, idle
threshold folds, and cache-TTL folds still run before that wait,
and the silence clock is not restarted.

## Verification

- bun test ./src/subagent/nudge-director.test.ts (48 pass, exit 0)
- bun run typecheck

Fixes CL-8912

Author: TheGreatAxios

docs/ARCHITECTURE.md

@@ -162,7 +162,7 @@ The ChatDirector counts consecutive assistant turns that contain tool calls and
 
 #### Sub-agent stall management
 
-`SubAgentDirector` tracks `lastActivityAt`, updated on every real `inference.done` and `tool.done`. Directors are pure `decide(event, ...)` functions with no timer of their own and the reactor has no proactive "idle" event, so a genuinely silent worker (e.g. parked on a long-running background command with nothing else to do) produces no event for the director to react to. `runSubAgent` (`src/subagent/index.ts`) arms an external interval, at `subAgentStallTimeoutMs`, that pings the same content-less continuation channel the compaction governor uses to re-enter an idle reactor (`requestContinuation`). The director only acts on a ping if the elapsed time since `lastActivityAt` has crossed the timeout. A ping can still be delivered while `execute_tools` is in flight; outstanding call ids from the last `inference.done` reset the silence clock and wait rather than recording `stall-nudge` with a huge `silenceMs`. The first stall past the timeout records `stallNudgeAt` and issues one continuation nudge (asking the worker to check on the background work or report status). Later empty pings inside `subAgentStallTimeoutMs` of that instant wait without stopping or treating the ping as activity; salvage fires only once a ping arrives after that grace with still no `tool.done` / turn-boundary reset. That grace is what keeps two queued interval ticks from salvaging hundreds of milliseconds after the nudge. Any real activity clears `stallNudgeAt`, so a worker that is genuinely working through a slow single turn is never penalized. After the worker has already replied with a terminal report (complete envelope or salvage), further empty continuations — idle-compact meter sync or stall pings — return `wait` instead of falling through to `DefaultDirector.infer`; only a non-empty parent message (`resume_agent` / `send_input`) re-opens the brief. A parked `ask_director` is the same class of wait: empty stall pings are dropped (not deferred) for the park duration, compact-continue hops skipped during the park flush after unpark, and `SubAgentDirector` returns `wait` on any empty continuation that still arrives while the ask is pending so a long park cannot start a billable infer.
+`SubAgentDirector` tracks `lastActivityAt`, updated on every real `inference.done` and `tool.done`. Directors are pure `decide(event, ...)` functions with no timer of their own and the reactor has no proactive "idle" event, so a genuinely silent worker (e.g. parked on a long-running background command with nothing else to do) produces no event for the director to react to. `runSubAgent` (`src/subagent/index.ts`) arms an external interval, at `subAgentStallTimeoutMs`, that pings the same content-less continuation channel the compaction governor uses to re-enter an idle reactor (`requestContinuation`). The director nudges only once elapsed time since `lastActivityAt` has crossed the timeout. A ping inside that window, or when stall timing is unset, returns `wait` and does not start a model turn or stamp `lastActivityAt`. A ping can still be delivered while `execute_tools` is in flight; outstanding call ids from the last `inference.done` reset the silence clock and wait rather than recording `stall-nudge` with a huge `silenceMs`. The first stall past the timeout records `stallNudgeAt` and issues one continuation nudge (asking the worker to check on the background work or report status). Later empty pings inside `subAgentStallTimeoutMs` of that instant wait without stopping or treating the ping as activity; salvage fires only once a ping arrives after that grace with still no `tool.done` / turn-boundary reset. That grace is what keeps two queued interval ticks from salvaging hundreds of milliseconds after the nudge. Any real activity clears `stallNudgeAt`, so a worker that is genuinely working through a slow single turn is never penalized. After the worker has already replied with a terminal report (complete envelope or salvage), further empty continuations — idle-compact meter sync or stall pings — return `wait` instead of falling through to `DefaultDirector.infer`; only a non-empty parent message (`resume_agent` / `send_input`) re-opens the brief. A parked `ask_director` is the same class of wait: empty stall pings are dropped (not deferred) for the park duration, compact-continue hops skipped during the park flush after unpark, and `SubAgentDirector` returns `wait` on any empty continuation that still arrives while the ask is pending so a long park cannot start a billable infer.
 
 **Intervention log**: every stop and nudge is appended as one JSONL record to `interventions.jsonl` in the firing worker's trace dir (`src/subagent/intervention-log.ts`), carrying the trigger's measured value beside the threshold it crossed, the provider/model/family it fired on, and the run state at that moment (turns used vs budget, tool calls, read/edit counts). A refused parent re-dispatch is recorded on the parent side, where no worker run exists to record it. The parent also appends one `outcome` record per completed dispatch — the salvage kind `classifyBriefSalvage` assigned, or a clean-complete marker, plus the dispatch count — so the log carries dispatch outcomes as well as interventions, and a stop record can later be read alongside what the dispatch it touched actually produced. Writes are fire-and-forget and swallow their own errors — a diagnostic must not be able to fail a run. `scripts/intervention-forensics.ts` aggregates these across local sessions: per-intervention counts by model family, the measured-value distribution against the threshold, two context columns (stops that fired on runs which had already edited files; stops that fired before half the turn budget was spent — neither is a measured false-positive rate, since either is equally consistent with a correct stop or a wrong one), and outcome counts by kind. This exists because every threshold in this tree was set by judgment and four of those judgments were later reverted — a threshold change is expected to cite this data (CL-6938).
 

src/subagent/nudge-director.test.ts

@@ -335,10 +335,11 @@ describe("SubAgentDirector tool failure recovery", () => {
     expect(resumedTexts).toHaveLength(1);
     expect(resumedTexts?.[0]).toContain("A tool call failed");
 
-    const later = inferAction(
+    const later = actions(
       await director.decide(messageReceived(""), longState, caps),
     );
-    expect(ephemeralTexts(later)).toBeUndefined();
+    expect(later).toEqual([{ type: "wait" }]);
+    expect(later.some((action) => action.type === "infer")).toBe(false);
   });
 
   test("recovery nudge appends to ephemeral turns already on the infer", async () => {
@@ -462,10 +463,11 @@ describe("SubAgentDirector tool failure recovery", () => {
     expect(resumedTexts).toHaveLength(1);
     expect(resumedTexts?.[0]).toContain("A tool call failed");
 
-    const later = inferAction(
+    const later = actions(
       await director.decide(messageReceived(""), state, caps),
     );
-    expect(ephemeralTexts(later)).toBeUndefined();
+    expect(later).toEqual([{ type: "wait" }]);
+    expect(later.some((action) => action.type === "infer")).toBe(false);
   });
 
   test("successful nudged infer then later overflow does not resurrect recovery", async () => {
@@ -1530,6 +1532,153 @@ describe("SubAgentDirector ask_director park wait-guard", () => {
   });
 });
 
+describe("SubAgentDirector idle stall ping", () => {
+  const STALL_NUDGE_TEXT =
+    "No activity has been observed for a while. If you are waiting on a " +
+    "background command, check its status now; otherwise continue working or " +
+    "write your report.";
+
+  test("empty ping inside the stall window waits and does not infer", async () => {
+    let now = 8_000_000;
+    const director = new SubAgentDirector(
+      "system",
+      [],
+      undefined,
+      1_000,
+      () => now,
+    );
+    const caps = createTestCapabilities();
+
+    await director.decide(inferenceDoneText("working"), state, caps);
+
+    now += 200;
+    const early = actions(
+      await director.decide(messageReceived(""), state, caps),
+    );
+    expect(early).toEqual([{ type: "wait" }]);
+    expect(early.some((action) => action.type === "infer")).toBe(false);
+    expect(early.some((action) => action.type === "checkpoint")).toBe(false);
+
+    // The in-window wait must not restart the silence clock. One stall
+    // timeout from the original activity still nudges, once.
+    now = 8_000_000 + 1_000;
+    const nudge = actions(
+      await director.decide(messageReceived(""), state, caps),
+    );
+    expect(nudge).toContainEqual({
+      type: "checkpoint",
+      message: "subagent-stall-nudge",
+    });
+    expect(ephemeralTexts(inferAction(nudge))).toEqual([STALL_NUDGE_TEXT]);
+
+    now += 200;
+    const grace = actions(
+      await director.decide(messageReceived(""), state, caps),
+    );
+    expect(grace).toEqual([{ type: "wait" }]);
+
+    now += 800;
+    const stopped = actions(
+      await director.decide(messageReceived(""), state, caps),
+    );
+    expect(stopped).toContainEqual({
+      type: "checkpoint",
+      message: "subagent-stalled",
+    });
+    expect(stopped.some((action) => action.type === "infer")).toBe(false);
+    expect(stopped.some((action) => action.type === "reply")).toBe(true);
+  });
+
+  test("outstanding post-compact infer still infers on an in-window empty ping", async () => {
+    let now = 9_000_000;
+    let continuations = 0;
+    const director = new SubAgentDirector(
+      "system",
+      [],
+      () => {
+        continuations++;
+      },
+      60_000,
+      () => now,
+    );
+    const caps = createTestCapabilities();
+
+    const compact = actions(
+      await director.decide(overflowError(), state, caps),
+    );
+    expect(compact).toEqual([
+      {
+        type: "compact",
+        compactor: "pruning-compactor",
+        reason: "context-overflow",
+      },
+    ]);
+    expect(continuations).toBe(1);
+
+    now += 200;
+    const resumed = actions(
+      await director.decide(messageReceived(""), state, caps),
+    );
+    expect(resumed.some((action) => action.type === "infer")).toBe(true);
+    expect(resumed.some((action) => action.type === "wait")).toBe(false);
+  });
+
+  test("idle-threshold fold still compacts on an in-window empty ping", async () => {
+    let now = 10_000_000;
+    let continuations = 0;
+    const director = new SubAgentDirector(
+      "system",
+      [],
+      () => {
+        continuations++;
+      },
+      60_000,
+      () => now,
+    );
+    const caps = createTestCapabilities();
+
+    await director.decide(inferenceDone(["read-1"]), longState, caps);
+    await director.decide(toolDone("read-1"), longState, caps);
+    const complete = actions(
+      await director.decide(
+        inferenceDoneText(REPORT_ENVELOPE, 999_999),
+        longState,
+        caps,
+      ),
+    );
+    expect(complete.some((action) => action.type === "reply")).toBe(true);
+    expect(continuations).toBe(1);
+
+    now += 200;
+    const folded = actions(
+      await director.decide(messageReceived(""), longState, caps),
+    );
+    expect(folded).toEqual([
+      {
+        type: "compact",
+        compactor: "pruning-compactor",
+        reason: "context-threshold",
+      },
+    ]);
+    expect(continuations).toBe(2);
+    expect(folded.some((action) => action.type === "infer")).toBe(false);
+    expect(folded.some((action) => action.type === "wait")).toBe(false);
+  });
+
+  test("no stall timeout waits on an unsolicited empty continuation", async () => {
+    const director = new SubAgentDirector("system", [], undefined);
+    const caps = createTestCapabilities();
+
+    await director.decide(inferenceDoneText("working"), state, caps);
+    const ping = actions(
+      await director.decide(messageReceived(""), state, caps),
+    );
+    expect(ping).toEqual([{ type: "wait" }]);
+    expect(ping.some((action) => action.type === "infer")).toBe(false);
+    expect(ping.some((action) => action.type === "checkpoint")).toBe(false);
+  });
+});
+
 function stubAdmission(
   notes: { provider: string; until: number }[],
 ): AdmissionQueue {

src/subagent/nudge-director.ts

@@ -343,6 +343,10 @@ export class SubAgentDirector extends DefaultDirector {
 
     const stallOutcome = this.checkStallPing(event, capabilities);
     if (stallOutcome !== null) return stallOutcome;
+    // Inside the stall window, or with no stall timeout, an empty
+    // continuation is not a model turn. Leave lastActivityAt and
+    // stallNudgeAt alone so the next real silence can still nudge.
+    if (isEmptyContinuation(event)) return capabilities.wait();
 
     // Keep the running local estimate current on every cycle (tool results and
     // rewrites included). Arming still happens inside noteInferenceDone, which
@@ -522,8 +526,9 @@ export class SubAgentDirector extends DefaultDirector {
    * stallNudgeAt. Queued pings that arrive inside the stallTimeoutMs grace
    * after that nudge neither stop nor restart the grace (and do not count as
    * activity). Stop only when a ping arrives after the grace with still no
-   * activity. Returns null when this event is not a stall check the director
-   * should act on (let it fall through as an ordinary continuation).
+   * activity. Returns null when this ping is not yet silence, or when stall
+   * timing is unconfigured. decide then waits on an empty continuation
+   * without stamping the silence clock, instead of inferring.
    */
   private checkStallPing(
     event: ReactorInboundEvent,