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/dashboard.rs

@@ -2141,6 +2141,7 @@ async fn post_cancel_reminder(
         Ok(0) => error_response(&format!("reminder {id} not pending (already delivered?)")),
         Ok(_) => {
             tracing::info!(%id, "operator cancelled reminder");
+            state.coord.emit_reminders_snapshot();
             (StatusCode::OK, "ok").into_response()
         }
         Err(e) => error_response(&format!("cancel reminder {id} failed: {e:#}")),
@@ -2160,6 +2161,7 @@ async fn post_retry_reminder(
         Ok(0) => error_response(&format!("reminder {id} not pending (already delivered?)")),
         Ok(_) => {
             tracing::info!(%id, "operator reset reminder failure for retry");
+            state.coord.emit_reminders_snapshot();
             (StatusCode::OK, "ok").into_response()
         }
         Err(e) => error_response(&format!("retry reminder {id} failed: {e:#}")),