fix(skills): clean up bash process groups on timeout and cancellation

[zakahan]

Calling bash_tool(command="sleep 900 | cat", timeout=5, ...) kills only the outer shell on timeout. The pipeline children retain the output pipes, so the subsequent unbounded process.wait() can prevent the tool from returning its timeout error.

Start commands in a separate session on POSIX and kill their process group on timeout or cancellation, including when the shell has already exited. Retain ownership when cancellation arrives during process creation. Limit cleanup waits to 5 seconds and close local transports if descendants still hold the pipes open. Preserve the tool signature, output formatting, timeout error, and cancellation propagation; non-POSIX systems retain direct-process termination.

The regression tests verify the original pipeline command returns a timeout error, its children stop, and the next call outputs TOOL_TIMEOUT_RECOVERED. They also cover cancellation during execution and spawning, an already-exited shell, cleanup deadlines, process-exit races, spawn failures, and normal output compatibility. Process-group cleanup does not claim to terminate descendants that explicitly detach into another session.

Validation:

• 77 tests passed: tests/tools/test_skills_bash_tool.py, tests/tools/test_skills_session_path.py, and tests/skills.
• pre-commit passed for both changed files (Ruff lint, formatting, and gitleaks).