Add WinUI 3 Expert

.github/plugin/marketplace.json

@@ -291,6 +291,12 @@
       "source": "typespec-m365-copilot",
       "description": "Comprehensive collection of prompts, instructions, and resources for building declarative agents and API plugins using TypeSpec for Microsoft 365 Copilot extensibility.",
       "version": "1.0.0"
+    },
+    {
+      "name": "winui3-development",
+      "source": "winui3-development",
+      "description": "WinUI 3 and Windows App SDK development agent, instructions, and migration guide. Prevents common UWP API misuse and guides correct WinUI 3 patterns for desktop Windows apps.",
+      "version": "1.0.0"
     }
   ]
 }