chore(backend): install AI agent skills for the .NET stack

Run autoskills in backend/: detects .NET/C#/ASP.NET Core/Minimal API and
installs 13 skills for universal (.agents) and claude-code (.claude) agents,
with backend/skills-lock.json. Adds ASP.NET Core, Minimal API + OpenAPI,
file-upload and containerization skills on top of the prior C#/.NET set.
Removes the stale root skills from the old layout.

backend/.agents/skills/aspnet-core/agents/openai.yaml

@@ -0,0 +1,5 @@
+interface:
+  display_name: "ASP.NET Core"
+  short_description: "[Windows only] Build and review ASP.NET Core web apps"
+  icon_large: "./assets/dotnet-logo.png"
+  default_prompt: "Create a new $aspnet-core website for me."