feat: 将插件 stdin 协议定为 v1，并按声明注入凭据

进程外插件改为只读环境变量和自身配置，避免读宿主 appsettings；宿主按 plugin.json 声明注入 LLM / OpenWeather 等凭据。

Plugins/weather/plugin.json

@@ -3,6 +3,7 @@
   "name": "WeatherAgent",
   "description": "按城市列表查询天气并汇总。",
   "version": "1.0.0",
+  "protocolVersion": 1,
   "timeoutSeconds": 180,
   "launch": {
     "command": "dotnet",
@@ -14,6 +15,12 @@
       "type": "openai-compatible",
       "required": true,
       "description": "由宿主注入 OpenAI 兼容的 endpoint / apiKey / model，不要写进本文件。"
+    },
+    {
+      "name": "weather",
+      "type": "openweather",
+      "required": false,
+      "description": "仅当本插件 appsettings.json 的 Weather:Provider 为 OpenWeather 时需要。Wttr 免费接口不用填。"
     }
   ],
   "inputs": [