v0.3.0: CI dotnet 8.0.424 pinned with isolated install

- Pin .NET SDK to 8.0.424 in all CI jobs to avoid floating channel drift

- Install SDK to runner temp via DOTNET_INSTALL_DIR to bypass

  C:\\Program Files\\dotnet overwrite/extract conflicts on Gitea Windows runners

- Simplify release job to dotnet publish + upload-artifact

- Exclude memory-bank/ via .gitignore (internal dev memory stays local)

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>