feat: OpenAI 引擎支持深度思考开关

新增:
- /engine.openai think [on|off|low|medium|high] 命令
- deep_think 配置项 (config.json)
- reasoning_effort 参数 (low/medium/high)

支持模型:
- DeepSeek: enable_thinking + reasoning_content
- OpenAI o-series: reasoning.effort
- 通用: reasoning_effort 参数

深度思考开启后:
- 自动注入思考参数到请求体
- 返回的思维链(reasoning_content)显示在终端
- info 命令显示当前思考状态

config.json.example

@@ -9,7 +9,9 @@
       "openai": {
         "api_key": "your-api-key-here",
         "base_url": "https://api.deepseek.com/v1",
-        "model": "deepseek-chat"
+        "model": "deepseek-chat",
+        "deep_think": false,
+        "reasoning_effort": "medium"
       }
     }
   },