A guide and an example for shipping onsystem with your design system #215

onsystem works best when the team that owns the design system sets it up once in its package, and every app that uses the package gets the checks with no config of its own. The docs don't lead with that yet, and no example shows it.

The plan:

• A guide for design-system teams: what goes in the package, what apps get, and how to roll it out.
• A short video of init in a package and an app picking it up.
• examples/design-system-monorepo (a packages/ui and an apps/web), which CI runs as a copy, as it does the shadcn demo.

Status: next. It follows the package-mode fixes in #205 and #202.