feat(gateway): notify(target, text, scope?) for PMs and Board

Pre-gateway PMs/Board sent formal notifications (require ack); gateway
had say/dm only. Now PMs and Board can issue ack-required notifications
via NotificationService through the standard envelope path.

tests/unit/gateway/test_content_actions_ownership.py

@@ -39,6 +39,7 @@ def _make_deps(**overrides: AsyncMock) -> ContentActionsDeps:
     a2a = overrides.get("a2a", AsyncMock())
     journal = overrides.get("journal", AsyncMock())
     workspace = overrides.get("workspace", AsyncMock())
+    notifications = overrides.get("notifications", AsyncMock())
     return ContentActionsDeps(
         task=task,
         git=git,
@@ -46,6 +47,7 @@ def _make_deps(**overrides: AsyncMock) -> ContentActionsDeps:
         a2a=a2a,
         journal=journal,
         workspace=workspace,
+        notifications=notifications,
     )