Add OSS maintenance audit command

[goonobu-dot]

Summary

• Add codex-maintainer-kit audit for OSS maintenance health scoring and prioritized next actions.
• Add an audit Markdown renderer, CLI wiring, tests, and generated example output.
• Document the audit-first maintainer workflow in README and CHANGELOG.

Test Plan

• python3 -m pytest -p no:cacheprovider tests -q
• PYTHONPATH=src python3 -m codex_maintainer_kit.cli audit . --output /tmp/oss-maintenance-audit.md

Notes

This is the first step toward a broader AI-assisted OSS maintenance toolkit: audit -> brief -> tasks -> human-reviewed merge.