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/agent_server.rs

@@ -811,6 +811,7 @@ pub(crate) fn store_remind(
         .store_reminder(agent, &stored_message, stored_path.as_deref(), due_at)
         .map_err(|e| format!("failed to store reminder: {e:#}"))?;
     tracing::info!(%id, %agent, %due_at, "reminder scheduled");
+    coord.emit_reminders_snapshot();
     Ok(())
 }