Implement async logging system that writes to /tmp/codex-wrapper-{pid}.log during execution and auto-deletes on exit.
- Add Logger with buffered channel (cap 100) + single worker goroutine
- Support INFO/DEBUG/ERROR levels
- Graceful shutdown via signal.NotifyContext
- File cleanup on normal/signal exit
- Test coverage: 90.4%
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Co-Authored-By: Claude <noreply@anthropic.com>
