Add automated release workflow

[JuanVqz]

Summary

• Adds .github/workflows/release.yml triggered when a PR from a release/ branch is merged into main
• Automatically tags the commit, extracts CHANGELOG notes, creates a GitHub release, builds the gem, and pushes to RubyGems
• Automates steps 6-9 of the manual release process

Steps automated

• Tag main with the version extracted from the branch name (release/v1.x.x -> v1.x.x)
• Push tag to GitHub
• Create GitHub release with CHANGELOG notes for that version
• Build gem and push to RubyGems

Prerequisites

• Add RUBYGEMS_API_KEY as a repository secret (Settings > Secrets > Actions)

Test plan

• Verify RUBYGEMS_API_KEY secret is set in repo settings
• Open a release/vX.Y.Z PR and merge it to confirm the workflow fires end-to-end

[JuanVqz]

@etagwerker I think this is sort of ready, but I'm wondering if you are interested in introducing it or if we can close it.

We will have the freedom to bump the versions as we want, since all that is still manual work; the only part is that any branch with the release prefix merged into main, is going to be cutting a new tag and publishing the gem into rubygems.