Support async agents

[cboos]

This is relatively new (seen in 2.1.34 today), and we kind of "support" it already, but it could be nicer.

Async agent launched successfully.
agentId: a8b740b (internal ID - do not mention to user. Use to resume later if needed.)
The agent is working in the background. You will be notified automatically when it completes.
Continue with other tasks.
output_file: /tmp/claude-1000/-home-cboos-.../tasks/a8b740b.output
To check progress before completion (optional), use Read or Bash tail on the output file.

<task-notification>
<task-id>a8b740b</task-id>
<status>completed</status>
<summary>Agent "Analyze relay.py coverage gaps" completed</summary>
<result>Now I'll analyze the uncovered lines and provide a comprehensive summary.

## Summary of Uncovered Lines in relay.py

### Coverage Analysis

The uncovered lines group into three main functions with varying levels of test coverage:
...
6. **Attachment handling** - Test file reading, base64 encoding, missing files, permission errors</result>
<usage>total_tokens: 23099
tool_uses: 2
duration_ms: 15506</usage>
</task-notification>
Full transcript available at: /tmp/claude-1000/-home-cboos-.../tasks/a8b740b.output