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/services/test_notification.py

@@ -159,7 +159,8 @@ async def test_send_blocker_notification(svc: NotificationService) -> None:
             from_agent="system",
             to_pm="cell-pm",
         )
-    assert any("Task t1" in row.subject for row in db.added)
+    # No task_title passed → falls back to the short-id display, not the raw id.
+    assert any("Task #t1" in row.subject for row in db.added)
 
 
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