Workspace draftable contract

Moved. The BelongsToWorkspace trait, the WorkspaceRegistry, and the assertWorkspaceDraftableContract Pest helper all live in the capell/publishing-studio add-on package.

➡ See docs.capell.app/publishing-studio-draftable-contract (canonical) or packages/publishing-studio/docs/publishing-studio-draftable-contract.md in capell-packages.