feat: add OpenAI client for fallback support

Implements ModelClient interface with OpenAI SDK to support GPT models
as fallback when local inference is unavailable. Includes tests with
mocked OpenAI API responses.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/models/index.ts

@@ -1,2 +1,3 @@
 export { AnthropicClient, type AnthropicClientConfig } from './anthropic.js';
+export { OpenAIClient, type OpenAIClientConfig } from './openai.js';
 export type { Message, ChatRequest, ChatResponse, ModelClient } from './types.js';