Fetches model metadata from the OpenRouter API and generates pkg/llm/registry_gen.go with typed ModelDefinition entries. Covers 9 providers: Anthropic, OpenAI, Google, xAI, Perplexity, Amazon, Meta, Mistral, DeepSeek. Signed-off-by: Aurélien Sibiril <81782+aureliensibiril@users.noreply.github.com>