docs: add plan metadata schema and lifecycle guidance ✨

**📝 Documentation**
- Add the canonical plan metadata schema, guide, and lifecycle management skill so plans have stable, schema-compliant frontmatter.
- Update planning and implementation prompts to require frontmatter, enforce pending/in-progress/completed transitions, and add a metadata-only update workflow.
- Add a new `update-plan-metadata` prompt to record verified lifecycle, GitHub, implementation, release, and validation facts without rewriting plan bodies.

**🧹 Chores**
- Allow the new `.github` markdown assets through `.gitignore` so the agent docs are tracked.
- Refresh `package.json` script usage to standardize on coverage testing and capture the latest manifest dependency versions.