Restructure template so solution files sit above the project folder

AGENTS.md, CLAUDE.md and the .slnx now live at the root of the
generated output, while the .csproj and other project-specific files
(.editorconfig, source files) stay nested inside a subfolder named
after the project.

Single dotnet new invocation now produces:

  ProjectName/
  ├── ProjectName.slnx
  ├── AGENTS.md
  ├── CLAUDE.md
  └── ProjectName/
      ├── ProjectName.csproj
      └── ...

Adds a post-action to register the generated project in the .slnx via
'dotnet sln add'. Verified end-to-end with a local dotnet new install.

.template.config/template.json

@@ -33,9 +33,17 @@
     }
   },
   "primaryOutputs": [
-    { "path": "LabApiPluginTemplate.csproj" }
+    { "path": "LabApiPluginTemplate/LabApiPluginTemplate.csproj" }
   ],
   "postActions": [
+    {
+      "description": "Adds the generated project to the nearest solution file, if one exists.",
+      "manualInstructions": [
+        { "text": "Run 'dotnet sln add' manually to add the project to your solution." }
+      ],
+      "actionId": "D396686C-DE0E-4DE6-906D-291CD29FC5DE",
+      "continueOnError": true
+    },
     {
       "description": "Restores the NuGet packages required by this project.",
       "manualInstructions": [