Migrate releases to bake-gem-github

[samuel-williams-shopify]

Move releases from the local gem:release command to reviewed release PRs and GitHub Actions publishing with RubyGems Trusted Publishing.

Replace the maintenance dependency with bake-gem-github, add its generated preparation, validation, and publishing workflows, and update the README release command. Preserve certificate signing and the release-notes/documentation version hook; remove the old GitHub release hook because the publisher now creates the release.

Generated with bake-gem-github 0.5.0. The proposed rules require the existing supported Ruby, lint, and coverage checks plus release validation, and two PR approvals with explicit administrator bypass. Once applied, these rules cover all PRs into main.

Validation:

• Full Sus suite: 240 tests, 1,193 assertions passed on Ruby 4.0.5.
• RuboCop passed for the changed Ruby and Markdown files.
• Release validation against origin/main and unsigned gem build passed.
• Generated YAML/JSON parsed successfully; setup regeneration produced no changes.
• Reviewed gem:github:setup:plan without applying remote settings.

Before the first release:

1. Create the rubygems GitHub environment restricted to main, and make the matching GEM_SIGNING_KEY available to the publishing job.
2. Configure the RubyGems Trusted Publisher for socketry/protocol-http2, workflow release-publish.yaml, environment rubygems.
3. After merging this setup and confirming the configured CI checks run, review bundle exec bake gem:github:setup:plan and apply the managed rules with bundle exec bake gem:github:setup:apply.

See the shared setup guide for publishing configuration and recovery.