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/.claude/skills/aspnet-core

@@ -0,0 +1 @@
+../../.agents/skills/aspnet-core

backend/.claude/skills/aspnet-minimal-api-openapi

@@ -0,0 +1 @@
+../../.agents/skills/aspnet-minimal-api-openapi

backend/.claude/skills/containerize-aspnetcore

@@ -0,0 +1 @@
+../../.agents/skills/containerize-aspnetcore

backend/.claude/skills/csharp-async

@@ -0,0 +1 @@
+../../.agents/skills/csharp-async

backend/.claude/skills/csharp-docs

@@ -0,0 +1 @@
+../../.agents/skills/csharp-docs

backend/.claude/skills/csharp-mstest

@@ -0,0 +1 @@
+../../.agents/skills/csharp-mstest

backend/.claude/skills/csharp-nunit

@@ -0,0 +1 @@
+../../.agents/skills/csharp-nunit

backend/.claude/skills/csharp-tunit

@@ -0,0 +1 @@
+../../.agents/skills/csharp-tunit

backend/.claude/skills/csharp-xunit

@@ -0,0 +1 @@
+../../.agents/skills/csharp-xunit

backend/.claude/skills/dotnet-best-practices

@@ -0,0 +1 @@
+../../.agents/skills/dotnet-best-practices

backend/.claude/skills/dotnet-design-pattern-review

@@ -0,0 +1 @@
+../../.agents/skills/dotnet-design-pattern-review

backend/.claude/skills/dotnet-upgrade

@@ -0,0 +1 @@
+../../.agents/skills/dotnet-upgrade

backend/.claude/skills/minimal-api-file-upload

@@ -0,0 +1 @@
+../../.agents/skills/minimal-api-file-upload