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>

plugins/external.json

@@ -76,6 +76,33 @@
       "path": "plugins/dotnet"
     }
   },
+  {
+    "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": "dotnet-diag",
     "description": "Skills for .NET performance investigations, debugging, and incident analysis.",