feat: add MiniMax as alternative LLM provider

[octo-patch]

Summary

• Add multi-provider LLM factory (llm_provider.py) with auto-detection from API keys
• MiniMax M2.7 / M2.7-highspeed supported via OpenAI-compatible API with temperature clamping
• New env vars: LLM_PROVIDER, LLM_BASE_URL, LLM_TEMPERATURE, MINIMAX_API_KEY
• Updated .env.example and README model table with MiniMax docs

Changes

File	Change
apps/agent/src/llm_provider.py	New provider factory with presets, auto-detection, temp clamping
apps/agent/main.py	Replace hardcoded ChatOpenAI() with create_llm() factory
.env.example	Add MiniMax API key and provider config docs
README.md	Add MiniMax M2.7 to model table + usage instructions
apps/agent/tests/	28 unit tests + 3 integration tests

How it works

Set MINIMAX_API_KEY in your .env and the provider is auto-detected. Defaults to MiniMax-M2.7 (1M context window).

Existing OpenAI usage is fully backward-compatible.

Test plan

• 28 unit tests covering provider detection, presets, factory, temperature clamping, edge cases
• 3 integration tests against real MiniMax API (basic completion, streaming, multi-turn)
• Manual verification with make dev using MiniMax M2.7 for generative UI