Bound ProcessRunner timeout across shell descendants

[chubes4]

Summary

• enforce ProcessRunner timeouts across shell descendants when launch-established tree containment is available
• use one immediate force signal for verified POSIX process groups and checked taskkill /T /F on Windows
• avoid descendant PID walking/signaling on unverified macOS/POSIX fallback paths
• expose additive cleanup containment and verification diagnostics in both WP_Error and error_as_result envelopes

Closes #930.

How to test

1. Run php tests/process-runner-command-spec.php.
2. Run php tests/worktree-staleness-fetch-timeout.php.
3. Run php -l inc/Support/ProcessRunner.php.
4. Run php -l tests/process-runner-command-spec.php.
5. Run composer validate --no-check-publish.

Compatibility

• Public signatures and normal command results are unchanged.
• Timeout result data gains an additive cleanup object.
• Linux with setsid and Windows use verified tree containment. macOS and POSIX hosts without launch-established containment terminate only the owned parent and report cleanup.verified: false; they no longer risk signaling unrelated recycled descendant PIDs.

AI assistance

• AI assistance: Yes
• Tool(s): OpenAI gpt-5.6-sol via OpenCode
• Used for: Diagnosed descendant timeout cleanup, implemented safe containment reporting, and added deterministic process tests; Chris reviewed and owns the change.