Add Ruby feature mapper

[inertia186]

Summary

Adds Ruby feature mapping across common Ruby project shapes:

• Detects Ruby projects from Gemfile, gems.rb, Rakefile, config.ru, gemspecs, and Ruby source trees.
• Maps Ruby project metadata, executables, Rake tasks, source groups, and test suites.
• Detects Bundler/Ruby defaults for test, lint, and package manager metadata.
• Adds Rails-specific mapping for app source, config, database files, views, assets, and tests.
• Adds Jekyll-specific mapping for site config, theme files, and content groups.
• Skips common Rails binstubs and avoids treating Rails app/assets as generic Node source.
• Updates README language support.

Validation

• corepack pnpm format:check
• corepack pnpm typecheck
• corepack pnpm test
• corepack pnpm lint
• corepack pnpm build

Notes

Tested the mapper read-only against several real Ruby repos, including Ruby gems, Jekyll sites, Rails apps, and mixed Rails/Node apps.