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/common/notifications.md

@@ -0,0 +1,33 @@
+## Notification preferences
+
+A background agent — **event triage** — reads every event that reaches this user (email, WhatsApp, calendar) and decides what is worth notifying. Its decisions are steered by `user-memory/notifications.md`: **that file is injected into event triage's prompt verbatim**, exactly as written. Event triage never sees this conversation, so this file is the only way the user's wishes reach it.
+
+When the user asks to change what they are notified about ("stop telling me about…", "ping me when…", "mute this chat"), **record it in `user-memory/notifications.md`**, in the user's own language.
+
+A rule is useful to event triage only if it can be matched against an event, so:
+
+- **Pin down the source when it matters.** Event triage sees each event's source (email, WhatsApp, calendar) and fields like sender, subject and chat name. "I don't want notifications from Mario" is ambiguous — Mario *where*? If the user didn't say and the answer changes the rule, ask. Rules about one source go under that source's heading.
+- **Some rules have no source.** "No promotional material" or "anything about the Guatemala trip" apply everywhere — file them under `## General`; no need to ask.
+- **Be as specific as you can.** An email address, a phone number or a chat name beats a first name. If memory holds the identifier (a contact note), use it.
+
+Keep the file in this shape — one rule per bullet, dated, edited in place rather than rewritten:
+
+```md
+# Notification preferences
+
+_Updated: YYYY-MM-DD_
+
+## General
+- No promotional material, except travel offers about Guatemala from "Viaggiare" or "Avventure nel mondo" — YYYY-MM-DD
+
+## Email
+- Always notify messages from sara@example.com (school) — YYYY-MM-DD
+
+## WhatsApp
+- Ignore group chats unless I am mentioned by name — YYYY-MM-DD
+
+## Calendar
+- Ignore events I created myself — YYYY-MM-DD
+```
+
+Create it with this skeleton if it doesn't exist yet. When you change it, update the `_Updated:_` line and keep `user-memory/index.md` in sync, as with any note. Keep this file for notification preferences only — anything else about the user belongs in its own note.