fix(auto): stop auto-mode when dispatch gap watchdog fails to dispatch (#537)

The dispatch gap watchdog is a one-shot timer that fires 5s after a unit
completes without a follow-up dispatch. Previously, if the watchdog's
dispatchNextUnit() call returned without actually dispatching a unit
(no sendMessage called), auto-mode was left permanently active but idle
— no new watchdog was started and no stopAuto was called.

This happened when:
- State between milestones had no dispatchable unit
- Stale completed-units.json after GSD updates caused skip loops
- dispatchNextUnit silently returned without finding work

Now the watchdog checks whether a unit was actually dispatched after its
retry attempt. If not, it stops auto-mode cleanly with a user-facing
message instead of leaving it stuck.

Closes #537

src/resources/extensions/gsd/auto.ts

@@ -325,6 +325,18 @@ function startDispatchGapWatchdog(ctx: ExtensionContext, pi: ExtensionAPI): void
         "error",
       );
       await stopAuto(ctx, pi);
+      return;
+    }
+
+    // If dispatchNextUnit returned normally but still didn't dispatch a unit
+    // (no sendMessage called → no timeout set), auto-mode is permanently
+    // stalled. Stop cleanly instead of leaving it active but idle (#537).
+    if (active && !unitTimeoutHandle && !wrapupWarningHandle) {
+      ctx.ui.notify(
+        "Auto-mode stalled — no dispatchable unit found after retry. Stopping. Run /gsd auto to restart.",
+        "warning",
+      );
+      await stopAuto(ctx, pi);
     }
   }, DISPATCH_GAP_TIMEOUT_MS);
 }