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/io/watcher.rs

@@ -75,6 +75,16 @@ pub enum WatcherEvent {
         /// Human-readable reason the story was blocked.
         reason: String,
     },
+    /// An agent hit a hard API rate limit and will be blocked until `reset_at`.
+    /// Triggers auto-scheduling of a timer and a notification with the resume time.
+    RateLimitHardBlock {
+        /// Work item ID the agent is working on.
+        story_id: String,
+        /// Name of the agent that hit the hard rate limit.
+        agent_name: String,
+        /// UTC instant at which the rate limit resets.
+        reset_at: chrono::DateTime<chrono::Utc>,
+    },
 }
 
 /// Return `true` if `path` is the root-level `.storkit/project.toml`, i.e.