Update alphaear skills to use MiniMax as default LLM

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

skills/alphaear-signal-tracker/scripts/utils/search_tools.py

@@ -563,7 +563,7 @@ class SearchTools:
             4. Return strictly JSON: {{"reuse": true/false, "index": <candidate_index_int>, "reason": "short explanation"}}
             """
             # 初始化模型
-            provider = os.getenv("LLM_PROVIDER", "ust")
+            provider = os.getenv("LLM_PROVIDER", "minimax")
             model_id = os.getenv("LLM_MODEL", "Qwen")
             host = os.getenv("LLM_HOST")
             if host: