Add GitHub Actions workflow for gem release via Trusted Publishing

[atm-snag2]

What

Closes #60

Why

How

• Adopted the same structure as qiita-markdown's release.yml
• Pinned actions/checkout and ruby/setup-ruby SHAs to match the existing test.yml

Out of scope

• Trusted Publisher configuration on rubygems.org (see issue TODO)
• Creating release environment on GitHub

Refs

[gemini-code-assist]

Note

Gemini is unable to generate a review for this pull request due to the file types involved not being currently supported.

[atm-snag2]

Verification

ruby -ryaml -e "YAML.safe_load(File.read('.github/workflows/release.yml')) && puts('YAML syntax: OK')"

YAML syntax: OK

• rspec / rubocop: sandbox constraint prevented local execution; CI will verify on push
• Workflow structure matches qiita-markdown release.yml (proven pattern)
• actions/checkout and ruby/setup-ruby SHAs match existing test.yml

[Umekawa]

LGTM