Add cancelled flag to prevent timed-out thread's finally block from
overwriting stdout/stderr when a subsequent execution has already
started capturing output.
- Add ThreadPoolExecutor in agent_worker for parallel request execution
- Add request_id correlation to prevent response mismatch between concurrent requests
- Add background listener thread per agent to dispatch responses to correct futures
- Add --timeout argument for hard request timeout (default: 120s from config)
- Remove signal handlers from terminal_manager, python_manager, tab_manager (use atexit only)
- Replace SIGALRM timeout in python_instance with threading-based timeout
This fixes requests getting queued behind slow operations and timeouts.
