Replace ImageEdit with ImageAgent for Qwen Image 2.0

Supports text-to-image, single-image edit and multi-image fusion (0-3
reference images) via a single tool. Renames imageEditModel config to
imageModel (default qwen-image-2.0) and adds imageWatermark toggle.

Co-Authored-By: Claude Opus 4.7 <[email protected]>

src/main/kotlin/JChatGPT.kt

@@ -848,8 +848,8 @@ object JChatGPT : KotlinPlugin(
         // 视觉代理
         VisualAgent(),
 
-        // 图像编辑模型
-        ImageEdit(),
+        // 图像生成与编辑
+        ImageAgent(),
 
         // 天气服务
         WeatherService(),