feat: live SSE updates for the SYST3M reminders section

Add RemindersChanged SSE event so the pending-reminders list in the
SYST3M tab updates live without polling.

Backend emission sites (every path that mutates the reminders table):
- agent_server: store_remind (remind MCP call)
- dashboard.rs: post_cancel_reminder, post_retry_reminder
- questions.rs: cancel_loose_end Reminder kind
- reminder_scheduler: after each delivery batch (any_delivered)

Coordinator gets emit_reminders_snapshot() mirroring the existing
emit_schedules_snapshot() pattern: lists PendingReminder rows from the
broker and emits DashboardEvent::RemindersChanged.

Frontend: applyRemindersChanged(ev) calls renderReminders(ev.reminders)
and is registered as reminders_changed in MUTATION_HANDLERS.

Docs: dashboard.md reminders_changed entry; CLAUDE.md file map updated.

docs/web-ui/dashboard.md

@@ -852,6 +852,14 @@ payload):
   re-renders `schedulesState` on receipt; tab activation still
   re-fetches as a safety net for approval-path inserts and
   disconnect windows.
+- `reminders_changed` (seq, reminders: `Vec<PendingReminder>`) —
+  full snapshot of all pending reminders. Emitted after every
+  reminder mutation: agent `remind` calls (`agent_server`),
+  operator cancel / retry (`/api/system/reminders/*`), `cancel_loose_end`
+  with Reminder kind, and the scheduler tick after each delivery
+  batch (`reminder_scheduler`). The SYST3M tab's reminders section
+  subscribes and calls `renderReminders` on receipt, so the list
+  updates live without polling.
 
 `/api/state` is **only fetched on cold-load and on the few
 forms that mutate non-event-derived state** (PURG3 +