feat(harness): add MiniMax provider to standalone Python LLM helper

[octo-patch]

Reason: Add MiniMax provider configuration to the standalone Python LLM helper so the portable harness can call MiniMax directly.

Changes

• .agent/harness/llm.py: add a minimax provider branch to llm_available() and call_model(). MiniMax is wired through its OpenAI- and Anthropic-compatible endpoints, so no new SDK dependency is required.
  • MINIMAX_REGIONS configures the global (api.minimax.io) and CN (api.minimaxi.com) regional endpoints with both openai_base_url and anthropic_base_url.
  • MINIMAX_MODELS registers the current MiniMax-M3 (1,000,000-token context window) and MiniMax-M2.7 (204,800-token context window) text models; MiniMax-M3 is the default.
  • Region is selected via AGENT_MINIMAX_REGION (default global_en); wire format via AGENT_MINIMAX_WIRE (openai default, or anthropic); API key via MINIMAX_API_KEY.
• .env.example: document the MiniMax environment variables.
• adapters/standalone-python/README.md: add MiniMax to the "Choose a provider" section.
• tests/test_llm_provider.py: add tests covering region/model config, provider availability, and the OpenAI and Anthropic wires for each region, plus error paths for unknown region/model/wire.

Checks

• python3 -m pytest tests/test_llm_provider.py -q -> 8 passed.
• Full suite: the new tests pass; pre-existing tests/test_mission_control.py failures are unrelated (a SyntaxError in harness_manager/mission_control_render.py on Python 3.10) and are not touched by this diff.

Note

Add MiniMax provider to standalone Python LLM helper

• Adds a minimax provider branch to llm.py, supporting two wire protocols: OpenAI-compatible and Anthropic-compatible, selected via AGENT_MINIMAX_WIRE (default: openai).
• Adds region routing via AGENT_MINIMAX_REGION (global_en or cn_zh), each mapping to distinct base URLs for both wire protocols.
• Supports two models (MiniMax-M3 with 1M context, MiniMax-M2.7 with 204K context), defaulting to MiniMax-M3 via AGENT_MODEL.
• Documents the new env vars in .env.example and README.md; adds full test coverage in test_llm_provider.py.
• Raises ValueError on unknown region, model, or wire protocol, failing fast on misconfiguration.

^{Macroscope summarized e1afa01.}