fix(FN-2054): harden agent loop stuck-detection and self-healing

- Add defensive cleanup for in-memory task tracking when tasks move state or agents pause
- Improve stuck-detection and self-healing flow to reduce leaked state and missed recovery paths
- Add logging around previously swallowed errors and tighten executor cleanup behavior
- Expand restart and self-healing reliability tests to cover regression scenarios

packages/engine/src/restart.integration.test.ts

@@ -738,6 +738,8 @@ describe("Scheduler after restart", () => {
     const todoTask = makeTask("FN-070", "todo");
     store.listTasks.mockResolvedValue([todoTask]);
     store.getSettings.mockResolvedValue({ ...DEFAULT_SETTINGS });
+    // Mock getTask for compare-and-swap verification in schedule()
+    store.getTask.mockResolvedValue(todoTask);
 
     // Needs parseFileScopeFromPrompt for overlap checks
     store.parseFileScopeFromPrompt.mockResolvedValue([]);
@@ -1320,6 +1322,8 @@ describe("Engine pause/unpause cycle", () => {
     store.listTasks.mockResolvedValue([todoTask]);
     store.getSettings.mockResolvedValue({ ...DEFAULT_SETTINGS, enginePaused: false });
     store.parseFileScopeFromPrompt.mockResolvedValue([]);
+    // Mock getTask for compare-and-swap verification in schedule()
+    store.getTask.mockResolvedValue(todoTask);
 
     const onSchedule = vi.fn();
     const scheduler = new Scheduler(store, {