Support asynchronous command hooks

[copyberry]

Support asynchronous command hooks

Why

Hook configurations can mark command handlers as asynchronous, but Codex previously skipped those handlers outside SessionEnd.

What changed

• Run asynchronous command hooks in the background with a per-session concurrency limit, while keeping SessionEnd hooks synchronous.
• Prevent asynchronous hooks from blocking, stopping, rewriting, or otherwise controlling the operation that launched them.
• Deliver warnings and additional context at safe turn boundaries: inject results into an active turn after sampling, or buffer them ahead of the next user prompt when the session is idle.
• Preserve in-flight hooks across configuration reloads, scope spilled output to the thread, and abort outstanding work during session shutdown.

Testing

Add unit and integration coverage for background scheduling, concurrency, output parsing, active and idle result delivery, configuration reloads, and shutdown cleanup.