[codex] Add npm release automation

[paladini]

Summary

• Add a tag-driven release workflow that validates the package, publishes missing npm versions, and creates or updates GitHub Releases.
• Add npm package metadata, release docs, changelog, and npm-first installation guidance.
• Extend SpecGov's own manifest so release/package files are governed by project docs.

Validation

• npm test
• npm run build
• npm run lint
• npm run typecheck
• npm run format:check
• git diff --check
• workflow YAML parse check
• npm pack --dry-run --json
• specgov check-pr --format json against this change set