Refine cgroup deletion behavior and logging

[seanwevans]

Motivation

• Avoid unsafe/unnecessary file unlinking when removing cgroup directories and instead rely on the kernel to manage tasks and files.
• Give callers clearer diagnostics when a cgroup cannot be removed by distinguishing busy/non-empty state from permission or missing-path errors.

Description

• Remove the loop that called Path.unlink() for each entry under the cgroup and only attempt path.rmdir() after a best-effort task drain.
• Implement a best-effort drain that reads child/cgroup.threads and writes thread IDs into the parent cgroup.threads to try to move tasks out before removal.
• Add errno-aware logging branches to distinguish FileNotFoundError, PermissionError, and busy/non-empty errors (errno.EBUSY / errno.ENOTEMPTY) from other OSError cases.
• Update tests in tests/test_cgroup.py to assert that delete() does not call Path.unlink() and that busy/non-empty and permission failures produce distinct warning messages.

Testing

• Ran pytest -q tests/test_cgroup.py and all tests passed (7 passed).

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Codex Task