feat: 新增方案 C 的 Handoff 路由宿主，并让抽城市支持自然语言。

把图 JSON 编译成 CreateHandoffBuilderWith + WithHandoff；Agent 固定 Id 以免交接工具名错位；DeepSeek 关闭 thinking，避免多轮丢掉 reasoning_content 导致 400。

MAF1.Core/Agents/Weather/WeatherAgent.cs

@@ -11,16 +11,30 @@ namespace MAF1.Agents.Weather;
 /// </summary>
 public static class WeatherAgent
 {
+    public const string DefaultDescription = "按城市列表查询天气并汇总。";
+
     /// <summary>创建带 GetWeather 工具的 Agent。instructions 要求每个城市都调工具，禁止编造气温。</summary>
-    public static AIAgent Create(AgentFactory factory, WeatherTools weatherTools)
+    public static AIAgent Create(
+        AgentFactory factory,
+        WeatherTools weatherTools,
+        string name = "WeatherAgent",
+        string? description = null,
+        string? extraInstructions = null)
     {
-        return factory.CreateAgent(
-            name: "WeatherAgent",
-            instructions:
+        string instructions =
             """
             你负责查询天气。用户给出的每个城市都必须调用一次 GetWeather，不要编造天气。
             用中文汇总所有城市的天气。
-            """,
+            """;
+        if (!string.IsNullOrWhiteSpace(extraInstructions))
+        {
+            instructions = instructions + "\n" + extraInstructions;
+        }
+
+        return factory.CreateAgent(
+            name: name,
+            instructions: instructions,
+            description: string.IsNullOrWhiteSpace(description) ? DefaultDescription : description,
             tools: [AIFunctionFactory.Create(weatherTools.GetWeather)]);
     }