AD-1002: Add contributor workflow

[alph-cmky]

Summary

• Document branch naming, commit style, and local verification commands in docs/contributing.md.
• Expand the pull request template with explicit verification, test, docs, and security-sensitive behavior checklists.
• Add GitHub Actions CI for install, lint, format, typecheck, test, and build.
• Update the AD-1002 backlog checklist.

Testing

• pnpm lint && pnpm format && pnpm typecheck && pnpm test && pnpm build

Stacked on #49.

Closes #25