Stability checkpoint — GH Actions backup CI + workflow rules .github path fix

- feat(ci): backup build-test workflow via GitHub Actions (npm ci + lint + unit + production build)
- fix(ci): add .github/** to workflow rules allowlist (symmetric with java per ADR-0070)

- ✅ Main pipeline #2518201631 success
- ✅ GH Actions CI #25719191695 success
- ✅ Auto-merge dev → main fires on .github changes