fix: keep slash commands working while a turn is running

`handleSubmit` treated every submission as guidance while a turn was running, including
the `/exit` command that `PromptInput` deliberately lets through. The CLI then queued
"/exit" as a user message and steered the turn instead of quitting.

Commands are not prompts, so they take the normal path again: non-exit commands stay
blocked while the turn runs, and `/exit` still quits.

Found by driving the real TUI in a pseudo-terminal against a stub OpenAI-compatible
server. The guidance path itself behaved as designed: the streaming answer was aborted
after 21 of 130 chunks, the partial text was kept and marked `meta.interrupted`, and the
follow-up request carried it together with the guidance as a user message.

Refs #113, #117

Author: mulyawansentosa

packages/cli/src/tests/prompt-input-queue.test.ts

@@ -139,6 +139,25 @@ test("PromptInput keeps blocking slash commands while busy", async () => {
   }
 });
 
+test("PromptInput still allows /exit while busy so the CLI can quit", async () => {
+  const harness = createHarness();
+  const submissions: PromptSubmission[] = [];
+  const app = renderPromptInput(harness, { busy: true, onSubmit: (submission) => submissions.push(submission) });
+  try {
+    await press(harness, app, "/exit");
+    await press(harness, app, "\r");
+    // App receives the command (not a prompt), so `/exit` keeps quitting the CLI
+    // instead of being queued as guidance for the running turn.
+    assert.deepEqual(
+      submissions.map((submission) => submission.command),
+      ["exit"]
+    );
+    assert.equal(submissions[0]?.steer, undefined);
+  } finally {
+    app.unmount();
+  }
+});
+
 test("PromptInput renders pending guidance and removes the last one on backspace", async () => {
   const harness = createHarness();
   let removed = 0;

packages/cli/src/ui/views/App.tsx

@@ -560,7 +560,9 @@ function App({ projectRoot, initialPrompt, resumeSessionId, forkSessionId, onRes
       // Prompts submitted while a turn is running become supplemental guidance:
       // SessionManager appends them as a user message right before the next LLM
       // call of that turn, so the model can revise what it is doing.
-      if (busyRef.current) {
+      // Slash commands are not prompts: `/exit` must still quit the CLI while a turn
+      // is running, exactly as it did before, so they take the normal path.
+      if (busyRef.current && !submission.command) {
         sessionManager.addSupplementaryPrompt(sessionManager.getActiveSessionId(), {
           text: submission.text,
           imageUrls: submission.imageUrls,