[LoopX smoke] Document an OpenAI-compatible API check

[xielixing]

This is a bounded end-to-end task for validating the LoopX MiniApp integration.

Acceptance criteria

• Add a README section named OpenAI-compatible API smoke test.
• Show a curl request to an OpenAI-compatible /chat/completions endpoint.
• Read the endpoint, API key, and model from OPENAI_BASE_URL, OPENAI_API_KEY, and OPENAI_MODEL environment variables.
• Never include a real API key or credential in the repository.
• Explain that a successful response returns HTTP 200 and text under choices[0].message.content.
• Keep the existing README heading intact.

The change should be documentation-only and easy to verify locally.