fix: terminate bash process groups on timeout

[lodyai]

Bash timeouts currently terminate only the shell, leaving descendants running and holding stdout/stderr pipes open. For example, sleep 6 & echo CHILD_PID=$!; wait with a one-second timeout took 6.07 seconds to return; the child was still alive after the shell received SIGTERM.

This change starts commands in a separate POSIX session and terminates the process group, allowing three seconds for SIGTERM before escalating to SIGKILL. Cleanup also handles descendants whose shell has already exited and bounds output draining to one second. Output finalization now propagates caller cancellation while preserving reader tasks for cleanup. bash.kill always uses termination, including when the shell leader has exited; cancellation and quit share the same cleanup implementation.

The bash tool description documents timeout cleanup and its limits. Descendants that create their own sessions are outside the group; Windows retains single-process termination. Background commands continue to ignore timeout_seconds.

Validation:

• Full pytest suite: 349 passed, 1 skipped on macOS / Python 3.14.
• Five new regression cases cover shell-first exit, SIGTERM-resistant descendants, and bounded output draining.
• Ruff lint and formatting, mypy, and git diff --check passed.

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	bub	1970ffb	Commit Preview URL Branch Preview URL	Sep 13 2026, 12:09 AM