feat(423): auto-schedule timer on rate limit to resume after reset

- pty.rs: detect rate_limit_event hard blocks, parse reset_at, emit
  WatcherEvent::RateLimitHardBlock with story_id, agent_name, reset_at
- watcher.rs: add RateLimitHardBlock variant to WatcherEvent enum
- timer.rs: add TimerStore::upsert (add-or-update-to-later) and
  spawn_rate_limit_auto_scheduler (listens for RateLimitHardBlock,
  upserts timer for the blocked story)
- notifications.rs: handle RateLimitHardBlock events with a debounced
  chat notification including the scheduled resume time;
  add format_rate_limit_hard_block_notification helper
- matrix/mod.rs: subscribe second watcher_rx for auto-scheduler,
  pass it to run_bot
- matrix/bot/run.rs: wire spawn_rate_limit_auto_scheduler into bot startup

Tests cover: AC1 (hard block detection in pty), AC2 (auto-scheduler
adds timer), AC3 (upsert deduplication), AC5 (chat notification sent),
AC6 (worktree preserved — timer fires start_agent on existing worktree)

server/src/chat/transport/matrix/bot/run.rs

@@ -22,6 +22,7 @@ pub async fn run_bot(
     config: super::super::config::BotConfig,
     project_root: PathBuf,
     watcher_rx: tokio::sync::broadcast::Receiver<crate::io::watcher::WatcherEvent>,
+    watcher_rx_auto: tokio::sync::broadcast::Receiver<crate::io::watcher::WatcherEvent>,
     perm_rx: Arc<TokioMutex<mpsc::UnboundedReceiver<crate::http::context::PermissionForward>>>,
     agents: Arc<AgentPool>,
     shutdown_rx: watch::Receiver<Option<crate::rebuild::ShutdownReason>>,
@@ -224,6 +225,11 @@ pub async fn run_bot(
         Arc::clone(&agents),
         project_root.clone(),
     );
+    // Auto-schedule timers when an agent hits a hard rate limit.
+    crate::chat::timer::spawn_rate_limit_auto_scheduler(
+        Arc::clone(&timer_store),
+        watcher_rx_auto,
+    );
 
     let ctx = BotContext {
         bot_user_id,