docs: Add NuGet.config setup and local build instructions (AGENTS.md §6)

- Document .NET SDK version mismatch & NuGet.config solution
- Add Release build with Development environment example
- Include stub API key setup for local Host startup
- Explain why Telerik source is included but not used

Closes: Local build failure on machines with preview SDK only
Verified: Both NuGet.config + appsettings prevent NU1507 errors

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

docs/SECRETS_LOCAL_DEVELOPMENT.md

@@ -67,6 +67,48 @@ ExternalApis:Kis:SecretKey = your-kis-secret-key
 
 ---
 
+## Build Issues & Solutions
+
+### .NET SDK Version Mismatch
+
+**Problem:** `global.json` requires .NET 10.0.100 GA, but only preview version installed
+```
+Requested SDK version: 10.0.100
+Install the [10.0.100] .NET SDK or update global.json to match an installed SDK.
+```
+
+**Solution:** Use NuGet.config to resolve package source conflicts
+```bash
+# NuGet.config at project root handles Telerik source override
+# (Telerik was configured in .sln but not actually used in code)
+# This prevents NU1507 "warning-as-error" during restore
+```
+
+The project includes `NuGet.config` which:
+- Configures only nuget.org as package source
+- Removes transitive Telerik source (build-only artifact)
+- Works with both GA and preview .NET 10 SDKs
+
+### Building Locally
+
+```bash
+cd C:\Job_Roomz\KArtSell.Aegis
+
+# Release build (optimized binaries)
+dotnet build KArtSell.sln -c Release
+
+# Development mode (with appsettings.Development.json)
+$env:ASPNETCORE_ENVIRONMENT = "Development"
+$env:KARTSELL_POSTGRES = "Host=127.0.0.1;Port=5432;Database=kartselldb_test;Username=kartsell_test;Password=kartsell4321@!_test"
+$env:KRX_OPENAPI = "stub-key-for-testing"
+
+dotnet run --project src/KArtSell.Host -c Release --no-build
+```
+
+Host listens on: `http://127.0.0.1:5002`
+
+---
+
 ## How It Works
 
 ### Development (dotnet run)