docs: add AGENTS.md guidance

[JustinBeckwith]

Summary

• add a repo-root AGENTS.md for Codex guidance in this repo
• document the actual npm, test, lint, and release conventions used here
• require Conventional Commits for commit messages and PR titles so release-please stays healthy
• call out the production-mode test behavior and the files that should stay aligned

Verification

• npm test
• npm run lint