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/"
+  ]
+}

plugins/winui3-development/README.md

@@ -0,0 +1,41 @@
+# WinUI 3 Development Plugin
+
+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.
+
+## Installation
+
+```bash
+# Using Copilot CLI
+copilot plugin install winui3-development@awesome-copilot
+```
+
+## What's Included
+
+### Commands (Slash Commands)
+
+| Command | Description |
+|---------|-------------|
+| `/winui3-development:winui3-migration-guide` | UWP-to-WinUI 3 migration reference with API mappings and before/after code snippets |
+
+### Agents
+
+| Agent | Description |
+|-------|-------------|
+| `winui3-expert` | Expert agent for WinUI 3 and Windows App SDK development. Prevents common UWP-to-WinUI 3 API mistakes, guides XAML controls, MVVM patterns, windowing, threading, app lifecycle, dialogs, and deployment. |
+
+## Key Features
+
+- **UWP→WinUI 3 API migration rules** — prevents the most common code generation mistakes
+- **Threading guidance** — DispatcherQueue instead of CoreDispatcher
+- **Windowing patterns** — AppWindow instead of CoreWindow/ApplicationView
+- **Dialog/Picker patterns** — ContentDialog with XamlRoot, pickers with window handle interop
+- **MVVM best practices** — CommunityToolkit.Mvvm, compiled bindings, dependency injection
+- **Migration checklist** — step-by-step guide for porting UWP apps
+
+## Source
+
+This plugin is part of [Awesome Copilot](https://github.com/github/awesome-copilot), a community-driven collection of GitHub Copilot extensions.
+
+## License
+
+MIT