feat(event-triage): user notification preferences via user-memory/notifications.md

The single-user notifications.md mechanism (a file in data/) died in the
multi-user move: assistant's prompt still pointed at it, but nothing read
it. Replace it with a memory note:

- event-triage injects user-memory/notifications.md verbatim on every
  pass and treats it as authoritative over its default heuristics
- a shared common/notifications.md fragment, included by assistant, kid
  and project-coordinator, tells the chat agents to record preference
  requests there: one dated rule per bullet under a source heading or
  General, asking for the source when ambiguous
- docs/system-agents.md explains the steering to users

agents/event-triage/meta.json

@@ -13,7 +13,7 @@
     }
   },
   "type": "system",
-  "inject_memory": ["user-memory/index.md"],
+  "inject_memory": ["user-memory/index.md", "user-memory/notifications.md"],
   "icon": "icon.png",
   "strength": "low"
 }