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/source-map.md

@@ -0,0 +1,43 @@
+# ASP.NET Core Source Map
+
+This skill is synthesized from the official ASP.NET Core documentation tree and overview pages. Use this file to map a task to the corresponding Microsoft Learn area before opening deeper docs.
+
+Core sources:
+
+- https://learn.microsoft.com/aspnet/core/
+- https://raw.githubusercontent.com/dotnet/AspNetCore.Docs/main/aspnetcore/toc.yml
+- https://github.com/dotnet/AspNetCore.Docs/tree/main/aspnetcore
+
+## Documentation Tree Mapping
+
+| ASP.NET Core docs area | Use this skill reference first |
+| --- | --- |
+| Overview, Get started, What's new | `stack-selection.md`, `versioning-and-upgrades.md` |
+| Fundamentals | `program-and-pipeline.md` |
+| Web apps | `ui-blazor.md`, `ui-razor-pages.md`, `ui-mvc.md` |
+| APIs | `apis-minimal-and-controllers.md` |
+| Real-time apps | `realtime-grpc-and-background-work.md` |
+| Remote Procedure Call apps | `realtime-grpc-and-background-work.md` |
+| Servers, Host and deploy | `testing-performance-and-operations.md` |
+| Test, Debug, Troubleshoot | `testing-performance-and-operations.md` |
+| Data access | `data-state-and-services.md` |
+| Security and Identity | `security-and-identity.md` |
+| Performance | `testing-performance-and-operations.md` |
+| Migration and updates | `versioning-and-upgrades.md` |
+
+## Areas To Consult Directly On Microsoft Learn
+
+The following topics are part of the ASP.NET Core documentation tree but are not expanded into their own dedicated reference file here:
+
+- globalization and localization
+- advanced hosting and YARP details
+- debugger and diagnostics tooling specifics
+- narrow API-reference pages for individual types
+
+When a task is dominated by one of those areas, go straight to the matching Microsoft Learn section after checking the reference files in this skill.
+
+## Practical Deep-Dive Rule
+
+- Start with the focused reference in this skill
+- If the task depends on a narrow platform detail, open the matching Learn article
+- If the task depends on version-specific behavior, confirm the correct moniker or breaking-changes page