fix(install): windows-safe liveness probe in runtime_status

[ketpatil77]

Description

Windows-safe liveness probe for headroom install status.

runtime_status() now checks PID liveness through a helper that avoids blindly calling os.kill(pid, 0) on Windows.

Closes #1746

Type of Change

• Bug fix (non-breaking change that fixes an issue)

Changes Made

• headroom/install/runtime.py: added _pid_alive() with psutil.pid_exists first, then a safe os.kill fallback that catches SystemError.
• headroom/install/runtime.py: runtime_status() now calls _pid_alive() instead of signalling the PID directly.
• tests/test_install/test_runtime.py: added regression tests for helper behavior and runtime status wiring.

Testing

• Unit tests pass (pytest)
• Linting passes (ruff check .)
• Type checking passes (mypy headroom)
• New tests added for new functionality
• Manual testing performed

Test Output

$ pytest tests/test_install/test_runtime.py
Focused runtime-status regression suite passed during local verification before PR submission.

$ ruff check headroom/install/runtime.py tests/test_install/test_runtime.py
Lint checks passed during local verification before PR submission.

Real Behavior Proof

• Environment: Windows local Headroom install-status flow.
• Exact command / steps: Exercised runtime-status helper path with Windows-safe PID liveness checks and regression coverage for fallback behavior.
• Observed result: runtime_status() no longer crashes on detached Windows PIDs and routes through helper logic instead of direct os.kill(pid, 0).
• Not tested: Cross-platform manual runtime install flow in this verification pass.

Review Readiness

• I have performed a self-review
• This PR is ready for human review

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have updated the CHANGELOG.md if applicable

[github-actions]

PR governance

This PR follows the template and is marked ready for human review.

[JerrettDavis]

Reviewed the runtime liveness change. The new helper avoids the Windows os.kill(pid, 0) crash path by preferring psutil when available, preserves a POSIX fallback, treats permission as live, and catches the Windows SystemError failure mode. The tests cover the helper and runtime_status wiring. Checks are green; this looks ready.

[ketpatil77]

Conflict rebase is done. Branch now sits on current main, keeps the Windows-safe liveness behavior, and re-runs clean locally: pytest tests/test_install/test_runtime.py -q (21 passed) and ruff check headroom/install/runtime.py tests/test_install/test_runtime.py.