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)

.config/dotnet-tools.json

@@ -0,0 +1,12 @@
+{
+  "version": 1,
+  "isRoot": true,
+  "tools": {
+    "vpk": {
+      "version": "0.0.1298",
+      "commands": [
+        "vpk"
+      ]
+    }
+  }
+}