Switch publish-release.yml to use rubykatzen/releaser publish-rubygems action

[ineedjet]

Context

rubykatzen/releaser#57 adds a reusable publish-rubygems composite action that encapsulates the setup-ruby → gem build → gem push flow.

Once that PR is released, baseline should replace its current inline gem block in .github/workflows/publish-release.yml:

# current — inline boilerplate
- uses: ruby/setup-ruby@v1
  with:
    ruby-version: "3.2"
- name: Build and push gem
  run: |
    rm -f rubykatzen-baseline-*.gem
    gem build baseline.gemspec
    gem push rubykatzen-baseline-*.gem
  env:
    GEM_HOST_API_KEY: ${{ secrets.RUBYGEMS_API_KEY }}

with a single step:

- uses: rubykatzen/releaser/.github/actions/publish-rubygems@vX.Y.Z
  with:
    api-key: ${{ secrets.RUBYGEMS_API_KEY }}

Acceptance Criteria

• Wait for feat: add publish-rubygems and publish-pypi reusable actions releaser#57 to be merged and released
• Replace inline gem publish block in publish-release.yml with the publish-rubygems action pinned to the new release tag
• Verify the next gem release publishes successfully to RubyGems.org