fix(gateway): notify_list/get/ack call NotificationDeliveryService (not Service)

Wave 1 wired notify_list/get/ack into ContentActions but pointed them at
`self.notifications` (which is NotificationService — sender side, with
send_blocker_notification / send_qa_ready_notification / etc.). The
read methods (list_for_agent, get_for_recipient_and_mark_read,
acknowledge) live on `NotificationDeliveryService` instead.

Smoke run 2026-05-11 surfaced this immediately:
  AttributeError: 'NotificationService' object has no attribute 'list_for_agent'

Fixes:
- roboco/api/deps.py — import NotificationDeliveryService and wire it
  in as a new ContentActionsDeps field `notification_delivery`.
- roboco/services/gateway/content_actions.py — add notification_delivery
  to ContentActionsDeps (Optional with `None` default for back-compat
  with any tests that don't supply it). Point notify_list, notify_get,
  notify_ack at self._deps.notification_delivery.
- tests/unit/gateway/test_content_actions.py — _make_deps adds a default
  AsyncMock for notification_delivery so existing tests continue to pass.

Quality: ruff + mypy clean. 505 unit tests pass.

tests/unit/gateway/test_content_actions.py

@@ -35,6 +35,7 @@ def _make_deps(**overrides: AsyncMock) -> ContentActionsDeps:
     journal = overrides.get("journal", AsyncMock())
     workspace = overrides.get("workspace", AsyncMock())
     notifications = overrides.get("notifications", AsyncMock())
+    notification_delivery = overrides.get("notification_delivery", AsyncMock())
     return ContentActionsDeps(
         task=task,
         git=git,
@@ -43,6 +44,7 @@ def _make_deps(**overrides: AsyncMock) -> ContentActionsDeps:
         journal=journal,
         workspace=workspace,
         notifications=notifications,
+        notification_delivery=notification_delivery,
     )