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/references/ui-mvc.md

@@ -0,0 +1,56 @@
+# MVC
+
+Primary docs:
+- https://learn.microsoft.com/aspnet/core/mvc/overview
+- https://learn.microsoft.com/aspnet/core/mvc/controllers/
+- https://learn.microsoft.com/aspnet/core/mvc/views/
+
+## Choose MVC When Actions And Views Matter
+
+Prefer MVC when the application benefits from explicit controllers, action-based routing, filters, view models, and a strong separation between orchestration and presentation.
+
+This is often the right fit for:
+
+- large server-rendered sites
+- applications with many cross-cutting filters or action conventions
+- applications that mix views and APIs in the same controller layer
+- teams already organized around controllers and views
+
+## Core Shape
+
+Enable MVC with views using:
+
+- `builder.Services.AddControllersWithViews();`
+- `app.MapControllerRoute(...)`
+
+Keep views focused on presentation. Keep controllers focused on HTTP orchestration. Put business rules in services.
+
+## Controller Guidance
+
+- Derive from `Controller` when the controller returns views
+- Keep actions small and explicit
+- Use model binding and validation instead of manual request parsing
+- Return view models, not EF entities, to views
+- Use POST-Redirect-GET for form submissions
+
+## View Guidance
+
+- Use layouts, partial views, and Tag Helpers to keep markup consistent
+- Keep complex display logic out of Razor markup when it becomes hard to follow
+- Use strongly typed view models
+- Avoid coupling views directly to persistence models
+
+## Structure And Scale
+
+- Use areas for large bounded sections such as Admin or BackOffice
+- Keep route conventions explicit
+- Apply filters when behavior truly belongs at the MVC layer
+- Avoid giant god controllers; split by cohesive feature or resource
+
+## Choosing MVC Over Razor Pages
+
+Prefer MVC over Razor Pages when:
+
+- multiple related actions share controller-level behavior
+- handler-level authorization or action filters matter
+- URL and action design are more natural than page-file routing