Add modernize-dotnet plugin (#1083)

* Add modernize-dotnet plugin

* Apply suggestions from code review

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

---------

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

.github/plugin/marketplace.json

@@ -248,6 +248,29 @@
       "description": "Comprehensive collection for building declarative agents with Model Context Protocol integration for Microsoft 365 Copilot",
       "version": "1.0.0"
     },
+    {
+      "name": "modernize-dotnet",
+      "description": "AI-powered .NET modernization and upgrade assistant. Helps upgrade .NET Framework and .NET applications to the latest versions of .NET.",
+      "version": "1.0.979-preview1",
+      "author": {
+        "name": "Microsoft",
+        "url": "https://www.microsoft.com"
+      },
+      "homepage": "https://github.com/dotnet/modernize-dotnet",
+      "keywords": [
+        "modernization",
+        "upgrade",
+        "migration",
+        "dotnet"
+      ],
+      "license": "MIT",
+      "repository": "https://github.com/dotnet/modernize-dotnet",
+      "source": {
+        "source": "github",
+        "repo": "dotnet/modernize-dotnet",
+        "path": "plugins/modernize-dotnet"
+      }
+    },
     {
       "name": "napkin",
       "source": "napkin",