Add Microsoft-maintained winui plugin (external reference) (#1686)

* Add winui plugin from microsoft/win-dev-skills

Adds the Microsoft-maintained WinUI 3 development plugin as an external
plugin reference. This is a Microsoft-managed repository so it qualifies
for the external-plugin policy exception in CONTRIBUTING.md.

The plugin provides agents and skills for WinUI 3 app development:
new app scaffolding, framework migration (WPF, WinForms, Electron,
Tauri, Flutter to WinUI 3), MSIX packaging, code signing, UI
automation testing, and Windows App SDK guidance.

Source: https://github.com/microsoft/win-dev-skills

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Remove winui3-development plugin (replaced by external winui plugin)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Nikola Metulev <nmetulev@microsoft.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.github/plugin/marketplace.json

@@ -708,10 +708,33 @@
       }
     },
     {
-      "name": "winui3-development",
-      "source": "winui3-development",
-      "description": "End-to-end WinUI 3 and Windows App SDK toolkit: expert agent, coding instructions, UWP-to-WinUI 3 migration guide, MVVM Toolkit reference, plus CLIs for packaging/debugging (winapp) and Microsoft Store publishing (msstore). Covers the full write → package → publish lifecycle for desktop Windows apps and prevents common UWP API misuse.",
-      "version": "1.2.0"
+      "name": "winui",
+      "description": "Agents and skills for WinUI 3 app development. Create new WinUI 3 desktop apps, convert from other frameworks (WPF, WinForms, Electron, Tauri, Flutter) to WinUI 3, or add features to existing WinUI 3 applications. Includes MSIX packaging, code signing, UI automation testing, and Windows App SDK guidance.",
+      "version": "0.3.0",
+      "author": {
+        "name": "Microsoft",
+        "url": "https://www.microsoft.com"
+      },
+      "homepage": "https://github.com/microsoft/win-dev-skills",
+      "keywords": [
+        "windows",
+        "winui",
+        "winui3",
+        "xaml",
+        "windows-app-sdk",
+        "msix",
+        "packaging",
+        "desktop",
+        "wpf-migration",
+        "electron-migration"
+      ],
+      "license": "MIT",
+      "repository": "https://github.com/microsoft/win-dev-skills",
+      "source": {
+        "source": "github",
+        "repo": "microsoft/win-dev-skills",
+        "path": "plugins/winui"
+      }
     }
   ]
 }