[codex] Fix Ruby Box test CI

[se4weed]

Summary

Adds a GitHub Actions workflow for the gem checks on a self-hosted ruby-box runner and fixes the existing Ruby::Box test failure that happens when child Ruby processes inherit Bundler's environment.

Root Cause

The Ruby::Box child-process tests were launched from bundle exec, so Bundler-related environment variables leaked into the child process. Under RUBY_BOX=1, the child then tried to boot through the parent bundle and failed while loading the project gemspec.

Changes

• Add a CI workflow that runs gem tests and Steep on a self-hosted ruby-box runner.
• Guard CI with a runtime check that Ruby::Box is available.
• Run Steep with --severity-level=error -j 1 so existing warnings do not fail CI and worker tmp cleanup stays stable.
• Wrap Ruby::Box child-process tests in Bundler.with_unbundled_env.
• Document the self-hosted runner requirement in English and Japanese READMEs.

Validation

• BUNDLE_PATH=/private/tmp/torikago-bundle bundle exec rake test
• BUNDLE_PATH=/private/tmp/torikago-bundle bundle exec steep check --severity-level=error -j 1
• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/ci.yml")'