Add OpenAI adapter

[simonhsze]

What

Add an OpenAI-compatible adapter (adapters/openai.py) that implements the LLMAdapter interface.

Why

Many users have OpenAI API keys. This is the most requested missing adapter.

How

1. Copy adapters/anthropic_api.py as a starting template
2. Implement call() using the openai Python SDK
3. Map ChatCompletion response to LLMResponse
4. Add lazy import in adapters/__init__.py (same pattern as other adapters)
5. Add tests using MockAdapter patterns (no real API calls)
6. Add openai to optional dependencies in pyproject.toml

Reference

• adapters/base.py — the interface to implement
• adapters/anthropic_api.py — closest reference implementation
• tests/test_adapters.py — test patterns