fix(notifications): task titles and agent slugs replace raw UUIDs (#616)

* fix(notifications): task titles and agent slugs replace raw UUIDs

Notification producers interpolated raw task/agent UUIDs into subjects and
bodies ('Task 68e1e4db-... unblocked', 'handed back to 00000000-...-0004').
A tiny notification_text helper (task_display: title-first with a #id8
fallback; agent_display: identity-map slug first, DB lookup fallback) now
feeds every producer: all 13 NotificationService methods, the 7
delivery-service bodies whose subjects were already title-based, the
substitute-PM ad-hoc insert, and the orchestrator/choreographer callers,
which thread the task row's title one call deeper. Fixes the literal
'cell_pm' role string sent as an agent slug in the merge-conflict
notification. Tool-call examples like unblock('<uuid>') keep the raw id on
purpose — agents need it.

* test(notifications): board-review subject assertion matches the humanized format

---------

Co-authored-by: Renn F <rennf93@users.noreply.github.com>

tests/unit/gateway/test_unblock_flip_breaker.py

@@ -97,7 +97,7 @@ async def test_third_unblock_notifies_ceo_once() -> None:
         env = await _unblock_once(c, pm_id, task_id, t)
         assert env.error is None, env.as_dict()
 
-    notify.assert_awaited_once_with(task_id, _THREE_FLIPS)
+    notify.assert_awaited_once_with(task_id, _THREE_FLIPS, t.title)
     assert markers.is_block_flip_notified(t) is True