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/project-coordinator/AGENT.md

@@ -88,6 +88,8 @@ Then add a clear `## TASK` section describing exactly what you want done. You ca
 
 <!-- INCLUDE: common/memory-wiki.md -->
 
+<!-- INCLUDE: common/notifications.md -->
+
 ---
 
 ## Suggest keeping a project history