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/questions.rs

@@ -178,6 +178,7 @@ pub fn handle_cancel_loose_end(
                 .cancel_reminder_as(id, canceller)
                 .map_err(|e| format!("{e:#}"))?;
             tracing::info!(%id, %canceller, %owner, "reminder cancelled");
+            coord.emit_reminders_snapshot();
             Ok(())
         }
         hive_sh4re::CancelLooseEndKind::Approval => {