Add RubyGems trusted publisher setup task

[samuel-williams-shopify]

Add bake gem:github:setup:publisher so maintainers can register an existing gem's RubyGems trusted publisher directly from its gemspec and config/release.yaml. The task reuses an exact matching registration, preserves other publishers, and reports API failures instead of treating them as missing configuration.

Reuse RubyGems login and MFA support with a scoped, 15-minute session key kept in memory, or an explicitly supplied API key. Add gem:github:setup:plan publisher=true to check registration; the default plan does not authenticate to RubyGems. Update the setup guide, distributed context, and release notes.

Automated validation uses mocked RubyGems requests and covers registration and reruns, repository/workflow/environment matching, authentication and MFA, API failures, configuration loading, and Bake argument parsing.

• Full test suite on Ruby 4.0.5: 192 tests, 748 assertions, 100% line coverage; covered:validate passes.
• RuboCop for all changed Ruby and Markdown files.
• Documentation coverage for lib: 41/41 public definitions.
• Manual validation: the maintainer successfully registered socketry/fiber-profiler on RubyGems using this task, with release-publish.yaml and the rubygems environment.