feat(424): rate-limit traffic-light dots and hard-block alerts

- Add HardBlock variant to WatcherEvent (story_id, agent_name, reset_time)
- In pty.rs, distinguish allowed_warning (throttle) from hard blocks;
  emit RateLimitWarning for throttles, HardBlock for actual 429s
- Add `throttled: bool` field to StoryAgent / AgentInfo
- Pool spawns a background listener that sets throttled=true on
  RateLimitWarning or HardBlock events and fires AgentStateChanged
- Status command shows traffic-light dots: ○ idle, ● running, ◑ throttled, ✗ blocked
- Read blocked flag from story front matter for the ✗ dot
- Notifications: RateLimitWarning silenced (too noisy); HardBlock sends
  urgent chat notification with optional reset time
- Tests added for traffic_light_dot, read_story_blocked, status output,
  and all notification paths

server/src/agents/mod.rs

@@ -188,6 +188,8 @@ pub struct AgentInfo {
     pub completion: Option<CompletionReport>,
     /// UUID identifying the persistent log file for this session.
     pub log_session_id: Option<String>,
+    /// True when a rate-limit throttle warning was received for this agent.
+    pub throttled: bool,
 }
 
 #[cfg(test)]