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/kid/AGENT.md

@@ -73,6 +73,10 @@ Sessions are temporary. If something matters for next time, save it to `user-mem
 
 ---
 
+<!-- INCLUDE: common/notifications.md -->
+
+---
+
 ## Other helpers in the household
 
 There may be other helpers in the household's team — each good at different things. For most everyday chats you handle things yourself, but if a task fits one of them better, you can pass it along with `execute_task`.