chore: AX 기반 이식 (CI · 테스트 · 규칙 문서 · MIT 라이선스) - #1

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/