feat: include subagent in task_metadata when resuming sessions

When delegate-task resumes a session via session_id, the response
task_metadata now includes a subagent field identifying which agent
was running in the resumed session. This allows the parent agent to
know what type of subagent it is continuing.

- sync-continuation: uses resumeAgent extracted from session messages
- background-continuation: uses task.agent from BackgroundTask object
- Gracefully omits subagent when agent info is unavailable

src/tools/delegate-task/sync-continuation.ts

@@ -128,7 +128,7 @@ ${result.textContent || "(No text output)"}
 
 <task_metadata>
 session_id: ${args.session_id}
-</task_metadata>`
+${resumeAgent ? `subagent: ${resumeAgent}\n` : ""}</task_metadata>`
    } finally {
      if (toastManager) {
        toastManager.removeTask(taskId)