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.

hive-c0re/src/reminder_scheduler.rs

@@ -63,6 +63,7 @@ fn tick(coord: &Arc<Coordinator>) {
     // Single-transaction batch: one DB lock acquisition for N reminders
     // instead of N sequential lock/unlock cycles.
     let results = coord.broker.deliver_reminders_batch(&items);
+    let any_delivered = results.iter().any(|r| r.is_ok());
     for ((id, agent, _body), result) in items.iter().zip(results.iter()) {
         if let Err(e) = result {
             let reason = format!("{e:#}");
@@ -82,6 +83,11 @@ fn tick(coord: &Arc<Coordinator>) {
             }
         }
     }
+    // Emit after the batch so the dashboard's pending-reminders list
+    // updates when deliveries land (removes delivered rows).
+    if any_delivered {
+        coord.emit_reminders_snapshot();
+    }
 }
 
 /// Build the inbox body for a due reminder. When `file_path` is None