Add OPENAI_IMAGE_USE_CHAT env var (true|false) to route image generation
through /chat/completions instead of /images/generations. Extracts base64
image data URLs from the chat response, enabling compatibility with
OpenAI-compatible providers that return images via chat API.
- Support --ref with OpenAI GPT Image models (gpt-image-1.5)
- Auto-select Google or OpenAI when --ref provided
- Change ref-related warnings to explicit errors with fix hints
- Add reference image validation before generation
- Improve retry logic to skip non-retryable errors