sm add must support Central Package Management (NU1008) when adding module PackageReferences

[antosubash]

Found during module-packaging Session 1 checkpoint.

The template Host uses Central Package Management (Directory.Packages.props). Adding <PackageReference Include="SimpleModule.FeatureFlags" Version="1.0.0" /> fails with NU1008; the version must go into a <PackageVersion> item instead.

sm add (Session 2) must detect CPM (ManagePackageVersionsCentrally / presence of Directory.Packages.props) and write the PackageVersion entry + version-less PackageReference; plain projects get the inline Version attribute.