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/mod.rs

@@ -90,8 +90,11 @@ pub fn spawn_bot(
 
     let root = project_root.to_path_buf();
     let watcher_rx = watcher_tx.subscribe();
+    let watcher_rx_auto = watcher_tx.subscribe();
     tokio::spawn(async move {
-        if let Err(e) = bot::run_bot(config, root, watcher_rx, perm_rx, agents, shutdown_rx).await
+        if let Err(e) =
+            bot::run_bot(config, root, watcher_rx, watcher_rx_auto, perm_rx, agents, shutdown_rx)
+                .await
         {
             crate::slog!("[matrix-bot] Fatal error: {e}");
         }