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)

.gitignore

@@ -0,0 +1,47 @@
+# .NET build outputs
+bin/
+obj/
+artifacts/
+publish/
+Releases/
+TestResults/
+
+# NuGet packages and local caches
+*.nupkg
+*.snupkg
+.nuget/
+
+# Local sandbox dirs created by scripts/release/*.ps1
+.dotnet/
+.appdata/
+
+# Code signing material - NEVER commit private keys
+*.pfx
+*.p12
+*.key
+*.pem
+*.cer
+
+# Secrets / local env
+.env
+*.env.local
+appsettings.*.local.json
+
+# OS
+Thumbs.db
+Desktop.ini
+.DS_Store
+
+# IDE
+.vs/
+.vscode/
+*.user
+*.suo
+*.userosscache
+
+# Local agent/tooling state (regenerable)
+.opencode/
+.codegraph/
+
+# Internal dev memory (stays local, not published)
+memory-bank/