BUGFIX: Startup crash when stored model provider is not authenticated

The CLI no longer raises RubyLLM::ConfigurationError at startup when the model saved in ~/.ruby_coded/config.yaml belongs to a provider that has no credentials on the current machine (e.g. switching computers with only Anthropic authenticated but a GPT model stored). Instead, the app falls back to the default model of the authenticated provider and shows an in-chat system message suggesting /login or /model to adjust.