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/coordinator.rs

@@ -389,6 +389,24 @@ impl Coordinator {
         });
     }
 
+    /// Emit a `RemindersChanged` snapshot event. Called from every
+    /// reminder mutation site (agent `remind` calls, operator cancel /
+    /// retry, and the scheduler after each delivery batch) so the
+    /// dashboard's pending-reminders list stays live without polling.
+    pub fn emit_reminders_snapshot(self: &Arc<Self>) {
+        let reminders = match self.broker.list_pending_reminders() {
+            Ok(rows) => rows,
+            Err(e) => {
+                tracing::warn!(error = ?e, "emit_reminders_snapshot: list failed");
+                return;
+            }
+        };
+        self.emit_dashboard_event(DashboardEvent::RemindersChanged {
+            seq: self.next_seq(),
+            reminders,
+        });
+    }
+
     /// Update the `step` label on a running queue entry and (if it
     /// actually changed) re-emit the queue snapshot so the dashboard
     /// renders the new phase. Returns `true` when the label was new