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_events.rs

@@ -202,6 +202,14 @@ pub enum DashboardEvent {
         seq: u64,
         schedules: Vec<hive_sh4re::WireSchedule>,
     },
+    /// Full snapshot of all pending reminders. Emitted after every
+    /// reminder mutation: agent `remind` calls, operator cancel / retry,
+    /// and the scheduler tick after each delivery batch. Lets the
+    /// dashboard's reminders section stay live without polling.
+    RemindersChanged {
+        seq: u64,
+        reminders: Vec<crate::broker::PendingReminder>,
+    },
 }
 
 impl DashboardEvent {
@@ -233,6 +241,7 @@ impl DashboardEvent {
             DashboardEvent::MetaUpdateRunning { .. } => "meta_update_running",
             DashboardEvent::RebuildQueueChanged { .. } => "rebuild_queue_changed",
             DashboardEvent::SchedulesChanged { .. } => "schedules_changed",
+            DashboardEvent::RemindersChanged { .. } => "reminders_changed",
         }
     }
 }
@@ -355,6 +364,10 @@ mod tests {
                 seq: 1,
                 schedules: Vec::new(),
             },
+            DashboardEvent::RemindersChanged {
+                seq: 1,
+                reminders: Vec::new(),
+            },
         ];
         for ev in samples {
             let v: serde_json::Value = serde_json::to_value(&ev).expect("serialise");