feat: add RubyGems registry support

[mvanhorn]

Summary

Adds gem:, ruby:, and rubygems: prefixes to fetch Ruby gem source code via rubygems.org API. Follows the existing registry pattern used by npm, PyPI, and crates.io.

Usage

opensrc path gem:rails
opensrc path ruby:sidekiq
opensrc path gem:nokogiri@1.16.0

Also adds --rubygems flag to opensrc clean and shows RubyGems in opensrc list.

Demo

How it works

The rubygems.org API (/api/v1/gems/{name}.json) returns source_code_uri which maps directly to a git clone URL. Falls back to homepage_uri if source_code_uri is absent, checking if it points to a known git host. Version-specific lookups use the v2 API (/api/v2/rubygems/{name}/versions/{version}.json).

normalize_repo_url (already in mod.rs) strips /tree/vX.Y.Z suffixes that rubygems.org sometimes appends to source_code_uri.

Changes

• rubygems.rs - New registry module: parse_rubygems_spec and resolve_rubygem following the existing pattern
• mod.rs - Add RubyGems variant to Registry enum, gem:, ruby:, rubygems: prefixes, extend parse_package_spec and resolve_package
• main.rs - Add --rubygems flag to clean command
• list.rs - Add RubyGems to the registries display and empty-state help text

Testing

3 new unit tests covering spec parsing (simple, versioned, hyphenated names). All 35 tests pass with --test-threads=1.

cargo test --manifest-path packages/opensrc/cli/Cargo.toml -- --test-threads=1
cargo clippy --manifest-path packages/opensrc/cli/Cargo.toml -- -D warnings
cargo fmt --manifest-path packages/opensrc/cli/Cargo.toml --check

This contribution was developed with AI assistance (Claude Code).

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Additional Suggestion:

The fallback registry scan in remove.rs is missing Registry::RubyGems, preventing removal of RubyGems packages without an explicit gem: prefix.