feat: add MiniMax provider support (M3 default)

[octo-patch]

Summary

Add MiniMax as a new LLM provider using its OpenAI-compatible API, with MiniMax-M3 as the default model.

Changes

• Core registry (core/providers/registry.py): Add ProviderSpec for minimax with openai_compat backend and default base URL https://api.minimax.io/v1
• Core config (core/config.py): Add minimax field to ProvidersConfig
• Nanobot registry (nanobot/nanobot/providers/registry.py): Add ProviderSpec with LiteLLM openai prefix and minimax base keyword detection
• Nanobot config (nanobot/nanobot/config/schema.py): Add minimax field to ProvidersConfig
• Example config (deepcode_config.json.example): Add minimax provider entry with ${MINIMAX_API_KEY}
• README: Add MiniMax to the provider configuration example; update model recommendations to use MiniMax-M3
• Tests (tests/minimax_provider_test.py): Unit tests covering registry lookup (including M3 and M2.7), config parsing, and provider instantiation

Configuration

{
  "agents": {
    "defaults": {
      "provider": "minimax",
      "model": "minimax/MiniMax-M3"
    }
  },
  "providers": {
    "minimax": {
      "apiKey": "${MINIMAX_API_KEY}"
    }
  }
}

Available Models

Model ID	Description
MiniMax-M3	512K context, up to 128K output, image input support (default)
MiniMax-M2.7	Previous generation model
MiniMax-M2.7-highspeed	Previous generation low-latency variant

API Reference

• Chat (OpenAI Compatible): https://platform.minimax.io/docs/api-reference/text-openai-api
• Pricing: https://platform.minimax.io/docs/guides/pricing-paygo