Update winui external plugin listing to v0.6.0 (#3000)

- Bump version 0.3.0 -> 0.6.0 to match the released win-dev-skills package.
- Point source.path at the canonical Agent Plugins 1.0 package (plugins/winui/agent-plugin) instead of the legacy compatibility root, so the Copilot Customize tab installs the portable package.
- Pin source.ref/source.sha to the immutable v0.6.0 tag (commit 68ae65d) for reproducible installs.
- Regenerate .github/plugin/marketplace.json via npm run build.

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

plugins/external.json

@@ -1418,7 +1418,7 @@
   {
     "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",
+    "version": "0.6.0",
     "author": {
       "name": "Microsoft",
       "url": "https://www.microsoft.com"
@@ -1441,7 +1441,9 @@
     "source": {
       "source": "github",
       "repo": "microsoft/win-dev-skills",
-      "path": "plugins/winui"
+      "path": "plugins/winui/agent-plugin",
+      "ref": "v0.6.0",
+      "sha": "68ae65d5c65ee87c3265a7f5abe3aaf97c7e6932"
     }
   }
 ]