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/versioning-and-upgrades.md

@@ -0,0 +1,51 @@
+# Versioning And Upgrades
+
+Primary docs:
+- https://learn.microsoft.com/aspnet/core/release-notes/
+- https://learn.microsoft.com/aspnet/core/release-notes/aspnetcore-10.0
+- https://learn.microsoft.com/aspnet/core/release-notes/aspnetcore-9.0
+- https://github.com/dotnet/AspNetCore.Docs/tree/main/aspnetcore/breaking-changes
+
+## Versioning Default
+
+- For new production apps in March 2026, prefer `net10.0`
+- For existing apps, match the repository's target framework unless the task is explicitly an upgrade
+- Before using a new API, confirm it exists in the target framework
+
+## Upgrade Workflow
+
+1. Identify the current target framework and SDK
+2. Read the "What's new" and breaking-changes pages for each version hop
+3. Compile and resolve obsoletions intentionally
+4. Re-run integration tests and auth flows
+5. Re-test deployment-specific behavior such as proxies, cookies, and static assets
+
+## High-Value Breaking-Change Checks
+
+When moving to ASP.NET Core 10, watch for:
+
+- cookie login redirects disabled for known API endpoints
+- `WithOpenApi` deprecation
+- `WebHostBuilder`, `IWebHost`, and `WebHost` obsolescence
+- Razor runtime compilation obsolescence
+
+When moving to ASP.NET Core 9, watch for:
+
+- `ValidateOnBuild` and `ValidateScopes` enabled in development when using `HostBuilder`
+- middleware constructor expectations and DI validation changes
+
+When moving to ASP.NET Core 8, watch for:
+
+- Minimal API `IFormFile` antiforgery requirements
+- `AddRateLimiter()` and `AddHttpLogging()` requirements when corresponding middleware is used
+
+## Migration Principles
+
+- Prefer migration to the modern hosting model when touching startup extensively
+- Remove compatibility shims only after tests confirm behavior
+- Avoid mixing new framework idioms with old startup architecture in a half-migrated state
+- Keep one authoritative target framework in project files unless multi-targeting is deliberate
+
+## Preview Feature Rule
+
+Do not introduce preview-only APIs or docs guidance unless the user explicitly asks for preview adoption or the repository is already on preview SDKs.