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.

frontend/packages/dashboard/src/tabs.js

@@ -227,6 +227,9 @@ window.marked = marked;
     schedulesState = (ev.schedules || []).slice();
     renderSchedulesList();
   }
+  function applyRemindersChanged(ev) {
+    renderReminders(ev.reminders || []);
+  }
   // Map from agent name → highest-priority in-flight queue entry
   // (`running` beats `queued`). Used by the container row renderer
   // to surface "building..." / "meta-updating..." badges on the
@@ -3604,6 +3607,7 @@ window.marked = marked;
     meta_update_running:     applyMetaUpdateRunning,
     rebuild_queue_changed:   applyRebuildQueueChanged,
     schedules_changed:       applySchedulesChanged,
+    reminders_changed:       applyRemindersChanged,
   };
   (function bindDashboardStream() {
     // Route through the SharedWorker so all open hyperhive tabs share