Add NuGet package packing and artifact upload to CI workflow

Automatically pack NuGet packages on successful builds and upload them as artifacts for easy download from the build details page. This only occurs on non-PR builds and Windows builds to avoid duplication.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: paulirwin

.github/workflows/dotnet.yml

@@ -29,3 +29,13 @@ jobs:
       run: dotnet build --no-restore
     - name: Test
       run: dotnet test --no-build --verbosity normal
+    - name: Pack NuGet packages
+      if: github.event_name != 'pull_request' && matrix.os == 'windows-latest'
+      run: dotnet pack --no-build --output nupkgs
+    - name: Upload NuGet packages
+      if: github.event_name != 'pull_request' && matrix.os == 'windows-latest'
+      uses: actions/upload-artifact@v4
+      with:
+        name: nuget-packages
+        path: nupkgs/
+        retention-days: 30