build: Add NuGet.config to resolve Telerik source (build-only, not used in code) (AGENTS.md §3)

- Add NuGet.config to override .sln-level package sources
- Telerik source was configured but not actually used (no PackageReference)
- Solution: Configure nuget.org as single source to avoid NU1507 warning-as-error
- Restores global.json allowPrerelease:false (GA SDK only, not preview)
- Enables local Release builds without SDK version conflicts

Fixes: Build failure on local machines with preview SDK 10.0.400
Verified: dotnet build KArtSell.sln -c Release → 0 errors ✅

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

NuGet.config

@@ -0,0 +1,7 @@
+<?xml version="1.0" encoding="utf-8"?>
+<configuration>
+  <packageSources>
+    <clear />
+    <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
+  </packageSources>
+</configuration>