feat: add self-hosted gem registries

[damacus]

Summary

• Extract RubyGems publishing into a tested composite-action script.
• Add Forgejo and Gitea RubyGems registry support alongside GitHub Packages, RubyGems.org, and Gem.coop.
• Add CI checks with ShellCheck, actionlint, YAML parsing, and stubbed publish tests.

Test Plan

• bash script/check
• git diff --check