Turn claims: token-scoped release, bound the CL-6670 wait

[TheGreatAxios]

Fixes CL-7129 — https://linear.app/abklabs/issue/CL-7129

Problem

createInMemoryTurnClaimStore.tryClaim (packages/chat/src/turn-claims.ts, pre-fix) let tryClaim return true again for a workbench whose claim was still held, once now() - claimedAt >= ttlMs. createWorkbenchTurnQueue.run (packages/chat/src/turn-queue.ts, pre-fix) holds a claim across its whole batch-drain loop, so a second run() call winning a fresh claim while the first was still draining started a second, concurrent drain loop over the same pendingByWorkbench queue — both loops able to pop and dispatch.

Separately, dispatchTurnBatch's agentTurns.waitUntilFree call (packages/chat/src/workbench-service.ts, pre-fix, CL-6670) ran with no bound of its own while holding this same claim. A slow prior turn for the same agent could itself hold the claim past its TTL and open exactly this window.

An earlier version of this PR bounded that wait with DEFAULT_WAIT_UNTIL_FREE_TIMEOUT_MS = CHAT_TURN_TIMEOUT_MS - DEFAULT_TURN_DISPATCH_TIMEOUT_MS - 30_000 (150s), below CHAT_TURN_TIMEOUT_MS (300s) — the documented max length a legitimate turn is allowed to run. A message queued behind a prior turn taking, say, 4 minutes would then time out waiting at 2.5 minutes and drop as undelivered: the exact bug CL-6670 fixed, reopened for any prior turn longer than ~2.5 minutes.

Change

• TurnClaimStore.tryClaim now returns an opaque token (or false) instead of a bare boolean. release and a new holds query both require the token, so a stale holder's release (after the TTL reassigns the claim) is a no-op and can never evict a live claim.
• createWorkbenchTurnQueue.run's drain loop calls holds() after every dispatch() and stops the moment it no longer holds the claim, leaving whatever queued behind it for the new holder to drain — never two loops draining the same workbench at once.
• dispatchTurnBatch wraps agentTurns.waitUntilFree in its own waitUntilFreeTimeoutMs (new, injectable). Its default, DEFAULT_WAIT_UNTIL_FREE_TIMEOUT_MS, is now CHAT_TURN_TIMEOUT_MS + 30_000 — it adds grace on top of the full legitimate turn length instead of subtracting from it, so a prior turn running the whole CHAT_TURN_TIMEOUT_MS no longer times out the wait.
• The claim TTL can no longer just be CHAT_TURN_TIMEOUT_MS: it has to clear this larger wait bound plus the dispatch deadline (DEFAULT_TURN_DISPATCH_TIMEOUT_MS) to stay an unreachable backstop. It's now its own constant, DEFAULT_TURN_CLAIM_TTL_MS = DEFAULT_WAIT_UNTIL_FREE_TIMEOUT_MS + DEFAULT_TURN_DISPATCH_TIMEOUT_MS + 30_000 (480s), derived from both rather than a second independent literal. apps/hub/src/index.ts's claim store and createChatRoutes's turnTimeoutMs both use it in place of CHAT_TURN_TIMEOUT_MS.
• AGENT_SECTION_MODE.turnTimeoutMs (standalone-launch.ts) is unaffected — it stays CHAT_TURN_TIMEOUT_MS, the platform's own per-occurrence turn timeout, unrelated to the claim TTL.
• TurnClaimStore has one implementer (createInMemoryTurnClaimStore); no other implementer needed updating.

Tests

• packages/chat/src/turn-claims.test.ts: tryClaim returns a token; release/holds are token-scoped; a release with a stale token (one the TTL already reassigned) is a no-op and never evicts the live holder.
• packages/chat/src/turn-queue.test.ts: a new test reproduces the reported shape — a message queues behind an in-flight turn, the claim TTL elapses mid-dispatch, a second run() wins a fresh claim and dispatches its own turn, and the first loop's own dispatch settling afterward no longer pops and re-dispatches what queued behind it; the second loop picks it up exactly once.
• packages/chat/test/agent-turn-dispatch.test.ts: a new test proves a prior turn that closes just under the injected wait budget still lets the queued send through, never posting an undelivered notice.
• packages/chat/src/workbench-service.timeouts.test.ts: new constants tests — DEFAULT_WAIT_UNTIL_FREE_TIMEOUT_MS clears CHAT_TURN_TIMEOUT_MS, and DEFAULT_TURN_CLAIM_TTL_MS clears DEFAULT_WAIT_UNTIL_FREE_TIMEOUT_MS + DEFAULT_TURN_DISPATCH_TIMEOUT_MS.
• bun test packages/chat/src/turn-queue.test.ts packages/chat/src/turn-claims.test.ts packages/chat/test/agent-turn-dispatch.test.ts packages/chat/test/turn-dispatch-deadline.test.ts packages/chat/src/workbench-service.timeouts.test.ts — all pass.
• bunx tsc --noEmit clean for packages/chat and apps/hub; eslint clean on changed files.