BIOP02-100 docs: README 재작성 — 정본(main) 기준 리포·브랜치 지도 + 하네스 게이트 반영 - #8

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, actions/setup-python@v5. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/