feat(metadata): add label backend and make project backend optional

[heoh]

Summary

Closes #6

This PR adds a new label metadata backend and makes project backend optional.
Default behavior is now label, so the action can work with GITHUB_TOKEN in most setups.

What changed

• Added LabelMetadataStore (src/github/label-metadata-store.ts)
• Added backend switch in inputs:
  • metadata-backend (label | project, default: label)
  • label-prefix (default: vc:)
  • label-default-color (default: b0b0b0)
  • project-tags-field-name (renamed from tags-field-name)
• Updated runtime wiring in src/handlers/shared.ts to select label/project store
• Updated input parsing/validation in src/context.ts
• Updated action.yml and README.md for new defaults and setup flow
• Updated build artifact (dist/index.js)

Compatibility notes

• Default backend changed to label
• If you want the old behavior, set metadata-backend: project
• For project backend, provide project-number (and usually PAT with repo, project)