Replace polyglot-test-agent with dotnet-test external plugin (#1685)

Remove all bundled polyglot-test-agent content (8 agents, 1 skill,
plugin metadata) and add dotnet-test as an external plugin from
dotnet/skills. The external plugin is a superset (11 agents, 22 skills).

Marketplace and doc indexes regenerated via npm run build.

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

.github/plugin/marketplace.json

@@ -212,6 +212,33 @@
         "path": "plugins/dotnet-diag"
       }
     },
+    {
+      "name": "dotnet-test",
+      "description": "Skills for running, writing, diagnosing, and migrating .NET tests: test execution, filtering, platform detection, coverage analysis, and MSTest workflows.",
+      "version": "0.1.0",
+      "author": {
+        "name": "Microsoft",
+        "url": "https://www.microsoft.com"
+      },
+      "homepage": "https://github.com/dotnet/skills",
+      "keywords": [
+        "dotnet",
+        "testing",
+        "mstest",
+        "xunit",
+        "nunit",
+        "test-generation",
+        "coverage",
+        "migration"
+      ],
+      "license": "MIT",
+      "repository": "https://github.com/dotnet/skills",
+      "source": {
+        "source": "github",
+        "repo": "dotnet/skills",
+        "path": "plugins/dotnet-test"
+      }
+    },
     {
       "name": "doublecheck",
       "source": "doublecheck",
@@ -499,12 +526,6 @@
       "description": "Comprehensive resources for building Model Context Protocol servers using the official PHP SDK with attribute-based discovery, including best practices, project generation, and expert assistance",
       "version": "1.0.0"
     },
-    {
-      "name": "polyglot-test-agent",
-      "source": "polyglot-test-agent",
-      "description": "Multi-agent pipeline for generating comprehensive unit tests across any programming language. Orchestrates research, planning, and implementation phases using specialized agents to produce tests that compile, pass, and follow project conventions.",
-      "version": "1.0.0"
-    },
     {
       "name": "power-apps-code-apps",
       "source": "power-apps-code-apps",