Set up automated releases with trusted publishing

Currently doing manual gem releases, would be nice to set up trusted publishing so we can just push a git tag and have GitHub Actions automatically publish to RubyGems without needing to store API keys anywhere.

The flow would be: bump version → commit → tag → push, then GitHub Actions picks it up, runs tests, and publishes the gem. Much cleaner and more secure than the current manual process.

Need to set up trusted publishing on RubyGems.org first (under API Access → Trusted Publishers), then create a GitHub Actions workflow that triggers on version tags.

**Links:**
- [RubyGems Trusted Publishing Guide](https://guides.rubygems.org/trusted-publishing/)
- [GitHub Actions Ruby Setup](https://github.com/ruby/setup-ruby)
- [RubyGems Release Action](https://github.com/rubygems/release-gem)

Start with the RubyGems Trusted Publishing Guide and inspect the repository's current release process, then review the ruby/setup-ruby and rubygems/release-gem workflows. Configure RubyGems trusted publishing and a GitHub Actions workflow triggered by version tags; done means tests run and the gem publishes without stored API keys.

Written by the indexing model from the issue text.