Add MiniMax M2.7 as an alternative LLM provider for the Planner & Coder
agents via OpenAI-compatible API.
Changes:
- Add request_minimax() and request_minimax_token() functions in
gpt_request.py using OpenAI SDK with MiniMax base URL
- Add minimax config entry in api_config.json
- Register minimax in agent.py API mapping and argparse choices
- Mention MiniMax in README.md LLM API section
- Add 16 unit tests and 3 integration tests
