Add WinUI 3 Expert (#865)

* WinUI agent

* Improvements

* Fixes

* Add WinUI 3 unit testing guidance across agent, instructions, and skill

- Expanded Testing section in agent with Unit Test App project setup,
  [TestMethod] vs [UITestMethod] attributes, and code examples
- Added Testing rules to instructions file
- Added Testing Migration section to migration guide skill with
  UWP-to-WinUI 3 test project mapping and updated checklist

Reference: https://learn.microsoft.com/en-us/windows/apps/winui/winui3/testing/create-winui-unit-test-project

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

* Update agents/winui3-expert.agent.md

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Copilot tweaks

* Update agents/winui3-expert.agent.md

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update agents/winui3-expert.agent.md

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

plugins/winui3-development/.github/plugin/plugin.json

@@ -0,0 +1,24 @@
+{
+  "name": "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",
+  "author": {
+    "name": "Awesome Copilot Community"
+  },
+  "repository": "https://github.com/github/awesome-copilot",
+  "license": "MIT",
+  "keywords": [
+    "winui",
+    "winui3",
+    "windows-app-sdk",
+    "xaml",
+    "desktop",
+    "windows"
+  ],
+  "agents": [
+    "./agents/winui3-expert.md"
+  ],
+  "skills": [
+    "./skills/winui3-migration-guide/"
+  ]
+}