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>

Author: 3 people

.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"
     }
   ]
 }