Publish projects-v2-sync as a versioned Marketplace action

## Summary

Release the action so consumers use `uses: alunduil/projects-v2-sync@v1`,
and list it on the Marketplace.

## Scope

- Release workflow tagging a moving major (`v1`) alongside semver tags.
- Marketplace listing (branding, categories) via `action.yml`.
- README: inputs, a minimal `uses:` example, and how to author a board
spec.

## Acceptance criteria

- [ ] `uses: alunduil/projects-v2-sync@v1` resolves to a working release.
- [ ] Marketplace listing published.
- [ ] README documents inputs + spec authoring.

## Additional context

- Depends on #3.
- Consumer wiring lands in alunduil/alunduil-infrastructure#90 — that
repo's `sync-project.yml` switches to `uses:` this action.

Start by checking dependency #3 and the current action.yml, then review the repository's release workflow and README. The consumer wiring is in alunduil/alunduil-infrastructure#90 at sync-project.yml. Done means the v1 reference resolves, the Marketplace listing is published, and the README documents inputs and board-spec authoring.

索引モデルが issue の本文から書いたものです。