refactor: 移除所有非ollama引擎，新增OpenAI兼容格式引擎

- 删除: mimo_engine, GLM_http_engine, qwq_engine, xfyun_spark_engine, iflow_api_engine, maas_http_engine, SDK_openAI
- 保留: ollama_engine (本地Ollama引擎)
- 新增: openai_engine (通用OpenAI API兼容引擎，支持DeepSeek/Grok/硅基流动/vLLM等)
- 新增: config.json.example 配置模板
- 更新: README.md / README.en.md 引擎说明

config.json.example

@@ -0,0 +1,20 @@
+{
+  "api": {
+    "engines": {
+      "ollama": {
+        "api_key": "",
+        "base_url": "http://localhost:11434",
+        "model": "qwen2.5:latest"
+      },
+      "openai": {
+        "api_key": "your-api-key-here",
+        "base_url": "https://api.deepseek.com/v1",
+        "model": "deepseek-chat"
+      }
+    }
+  },
+  "system": {
+    "default_engine": "ollama",
+    "max_history": 50
+  }
+}