Changes:
- Added 'minimax' provider to LLM factory
- Changed default LLM_PROVIDER from 'ust' to 'minimax'
- Changed default LLM_MODEL from 'Qwen' to 'MiniMax-Text-01'
- Updated REASONING_MODEL_PROVIDER and TOOL_MODEL_PROVIDER to minimax
- Sentiment tools now prefer MINIMAX_API_KEY over UST_KEY_API
- .env.example updated with MiniMax defaults
- Load credentials from ~/.config/opencode/.env automatically
- No need to configure .env in each skill directory
- Fixes issue where new skills couldn't find credentials
