Upload CLI package artifacts

[rndv1]

Summary

• upload the packed SnapForge .nupkg from the Build workflow
• keep artifacts short-lived with a 7 day retention window
• document how maintainers can use CI package artifacts for branch verification

Verification

• dotnet build --no-restore --configuration Release
• dotnet test --no-build --configuration Release --verbosity normal
• dotnet pack src/SnapForge.Cli/SnapForge.Cli.csproj --no-build --configuration Release --output artifacts/packages
• git diff --check origin/main...HEAD