feat(notifications): NotificationCoordinator skeleton + type registry

[tempus2016]

Summary

• Adds 6 notification type ID constants to const.py (Task 4, Step 1)
• Creates coord_notifications.py with NotificationCoordinator, NOTIFICATION_TYPES registry, _SafeDict template renderer, and full fire() dispatch pipeline
• Wires NotificationCoordinator into TaskMateCoordinator.__init__ as self.notifications
• 5 smoke tests covering: master-disabled skip, per-recipient enable/disable routing, safe template rendering with missing keys, bus event emission with recipients list, unknown-type logging

Test plan

• pytest tests/test_notifications_dispatch.py -v — 5/5 pass
• Full suite pytest -q — 417 passed, 0 failures
• Only 4 files changed: const.py, coord_notifications.py, coordinator.py, tests/test_notifications_dispatch.py