Add support for OpenAI, Anthropic, Ollama, and OpenAI-compatible
endpoints alongside the existing NEAR AI backend. Users can now
bring their own API keys via environment variables (LLM_BACKEND,
OPENAI_API_KEY, ANTHROPIC_API_KEY, etc.) while NEAR AI remains
the default.
Co-authored-by: Claude Opus 4.6 <[email protected]>
