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.

CLAUDE.md

@@ -82,7 +82,8 @@ hive-c0re/         host daemon + sibling operator CLI (lib + 2 bins)
                          (`ApprovalAdded` / `ApprovalResolved`,
                          `QuestionAdded` / `QuestionResolved`,
                          `TransientSet` / `TransientCleared`,
-                         `RebuildQueueChanged`, `SchedulesChanged`).
+                         `RebuildQueueChanged`, `SchedulesChanged`,
+                         `RemindersChanged`).
                          Each frame carries a
                          monotonic per-process `seq` clients use to
                          dedupe against snapshot reads.