Add local models registry config and rubyrt models command

[kplawver]

Summary

• Adds a models_file config option (also RUBYRT_MODELS_FILE env var) that points to a local RubyLLM models registry JSON. When set and the file exists, reviews load model metadata from there instead of the registry shipped with the ruby_llm gem, which can lag behind new provider models.
• Adds a rubyrt models command that refreshes the registry (RubyLLM.models.refresh!) and persists it to the configured path (RubyLLM.models.save_to_json), with an optional --path flag for one-off overrides. Creates parent dirs as needed and exits with a usage message when no path is configured.
• Extracts LlmClient.apply_provider_config! as a reusable class method so the models command can expose provider credentials to refresh! without duplicating the provider-key mapping.

Test plan

• bundle exec rspec — full suite passes (116 examples, 0 failures).
• Smoke-tested rubyrt models with no config (exits 1 with usage message) and with --path (refreshes, writes the JSON, creates nested dirs, reports model count).
• RuboCop is clean on every touched file; pre-existing offenses in debug_output.rb/verifier.rb/specs are unchanged and out of scope.

[rubyrt]

RubyRT Code Review

✅ No issues found across 13 file(s).