feat(FN-5346): add post-completion defensive backstop and shared reconcile

The merge adds a post-completion defensive backstop that probes and removes stale same-task `activeSessionRegistry` entries on `done`/`archived` transitions, completing FN-5346 with a shared reconcile helper, a defensive ownership probe wired into paused cleanup, audit event alignment, and regressio

Fusion-Task-Id: FN-5346

packages/engine/src/__tests__/executor-worktree-conflict.test.ts

@@ -28,7 +28,7 @@ describe("FN-4973: executor worktree conflict cleanup", () => {
     expect(activeSessionRegistry.lookupByPath(CONFLICT_PATH)).toBeNull();
     expect(store.logEntry).toHaveBeenCalledWith(
       "FN-4973",
-      "Cleared stale self-owned activeSessionRegistry entry",
+      "Cleared stale self-owned active-session entry before remove",
       CONFLICT_PATH,
     );
   });