Switch agent model to Anthropic Claude Fable 5

- New src/model.py factory: ChatAnthropic, default claude-fable-5,
  LLM_MODEL env override preserved (red-green tested)
- Drop langchain-openai + openai deps; add langchain-anthropic
- ANTHROPIC_API_KEY replaces OPENAI_API_KEY everywhere: render.yaml,
  Makefile, README, CONTRIBUTING, CLAUDE.md, docs (getting-started,
  deployment, bring-to-your-app, agent-state, agent-tools)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>