feat(FN-2866): wire provider-backed notification service into engine

- Add notification service module with provider abstractions and ntfy provider implementation
- Refactor NtfyNotifier into a compatibility wrapper that delegates task-event delivery to NotificationService
- Initialize and stop NotificationService from ProjectEngine while preserving gridlock notifications via NtfyNotifier
- Export notification APIs from engine index and add focused unit coverage for provider, service, and project-engine wiring

packages/engine/src/__tests__/project-engine.test.ts

@@ -72,6 +72,14 @@ vi.mock("../notifier.js", () => ({
   NtfyNotifier: vi.fn().mockImplementation(() => ({
     start: vi.fn(async () => undefined),
     stop: vi.fn(),
+    notifyGridlock: vi.fn(),
+  })),
+}));
+
+vi.mock("../notification/index.js", () => ({
+  NotificationService: vi.fn().mockImplementation(() => ({
+    start: vi.fn().mockResolvedValue(undefined),
+    stop: vi.fn(),
   })),
 }));