fix: subagent title extraction race condition

- Add 100ms debounce for new file detection to allow content to be written
- Add subagent:updated event for retroactive description updates
- Extract description in processEntry when first user message is processed
- Add extractDescriptionFromFile helper with retry in file change handler
- Update SubagentTranscriptEntry.content type to support string | array
- Add test coverage for subagent:updated event

Fixes 43% failure rate where subagents displayed raw IDs instead of descriptions
due to race condition when files were discovered before first line was written.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/web/server.ts

@@ -303,6 +303,10 @@ export class WebServer extends EventEmitter {
       this.broadcast('subagent:discovered', info);
     });
 
+    subagentWatcher.on('subagent:updated', (info: SubagentInfo) => {
+      this.broadcast('subagent:updated', info);
+    });
+
     subagentWatcher.on('subagent:tool_call', (data: SubagentToolCall) => {
       this.broadcast('subagent:tool_call', data);
     });