Prepare trusted NuGet publishing

[magiccodingman]

What changed

• configures shared NuGet metadata for the repository
• publishes only MagicSettings and MagicSettings.Server
• keeps MagicSettings.Share non-packable
• bundles MagicSettings.Share.dll inside both public packages
• suppresses a standalone MagicSettings.Share package dependency
• adds a tag-triggered trusted-publishing workflow using the release environment
• validates that exactly two packages are produced and that both contain the shared assembly
• uploads generated package artifacts before publishing

Why

The shared project is an implementation detail used by both public packages. Consumers should receive it automatically rather than installing a third standalone package.

GitHub setup still required

• create an Actions environment named release
• add the repository secret NUGET_USER with the NuGet profile username
• confirm the NuGet trusted-publishing policy authorizes publish-nuget.yml and the release environment

Validation

The existing CI workflow will build and test the solution on this pull request. The publishing workflow additionally validates package count and package contents when a v* release tag is pushed after merge.

I could not run a local pack in this execution environment because outbound GitHub access and the GitHub CLI were unavailable, so the PR is intentionally left as a draft until CI confirms the build.