Implement provider adapter layer (direct API or via LiteLLM)

[Aureliolo]

Context

Based on the LiteLLM evaluation (#4), implement the actual provider adapter layer. This is the concrete implementation of the unified interface designed in #3, connecting the framework to real LLM APIs.

Acceptance Criteria

• Anthropic provider working (Claude models)
• At least one additional provider working (OpenRouter or Ollama)
• Implements the unified interface from Design unified provider interface (base classes, protocols) #3
• Provider configuration loaded from YAML config files
• API keys sourced from environment variables
• Full async support for all provider operations
• Integration tests with mock/recorded API responses
• Error handling for: rate limits (429), authentication errors, timeouts
• Clean separation between provider-specific and generic code

Dependencies

• Depends on Evaluate LiteLLM: integration prototype, limitations, alternatives #4 (LiteLLM evaluation and decision)

Design Spec Reference

Section 9.1, 9.3 — Provider Interface and LiteLLM Integration