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/pool/lifecycle.rs

@@ -271,6 +271,7 @@ impl AgentPool {
                     project_root: Some(project_root.to_path_buf()),
                     log_session_id: Some(log_session_id.clone()),
                     merge_failure_reported: false,
+                    throttled: false,
                 },
             );
         }