Implements IAnswerModel over Anthropic's streaming Messages API: SSE
parsing (text_delta/usage/error), 429/529 retry with Retry-After or
exponential backoff, and a pure AnswerPrompt.Build for the evidence
and judgements prompt shape. Health check hits GET /v1/models.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>