Add native provider adapters (Anthropic, Gemini, Ollama)

[devin-ai-integration]

The Provider enum in crates/providers/src/lib.rs (lines ~43-48) only supports Mock, OpenAI, and Proxy despite the multi-LLM positioning in the README roadmap.

Problem

• The multi-LLM promise from the README roadmap is not yet delivered; only Mock/OpenAI/Proxy exist.

Suggested direction

• Add native provider adapters for Anthropic, Gemini, and Ollama.
• Since OpenAI/Proxy already use genai, the same abstraction can be extended to these providers.

References

• crates/providers/src/lib.rs — Provider enum (lines ~43-48)