Migrated internal/llm from manual net/http implementation to anthropic-sdk-go v1.47.0
and openai-go/v3 v3.39.0, reducing ~400 lines of retry, request-building, and streaming
code. Simplified LLMClient interface to single CompletionsWithCtx method. Fixed Anthropic
auth to use WithAuthToken (Bearer) instead of WithAPIKey (X-Api-Key) for proxy compatibility.
Fixed OpenAI message builder to use ExtractText() for proper []ContentBlock handling.
