feat: switch notification backend from Redis to Firestore

- Refactor FirestoreNotificationBackend to use time-gated window
  (window_hours) instead of notified_by_agent status filtering;
  mark_as_notified is now a no-op (agent is awareness-only).
- Update agent.py to instantiate FirestoreNotificationBackend using the
  shared firestore_db client instead of RedisNotificationBackend.
- Remove redis_host/redis_port settings from config.py; add
  notifications_collection_path, max_to_notify, and window_hours.
- Move redis/json imports inside RedisNotificationBackend methods so
  redis is only required if that backend is explicitly instantiated.
- Add utility scripts for checking and registering notifications.
- Add google-cloud-firestore dependency to pyproject.toml.

pyproject.toml

@@ -14,6 +14,7 @@ dependencies = [
     "pydantic-settings[yaml]>=2.13.1",
     "google-auth>=2.34.0",
     "google-genai>=1.64.0",
+    "redis>=5.0",
 ]
 
 [build-system]