feat: Replace example tests in Vercel Sandboxes (#11662)
## Summary
- Replaces the old example testing infrastructure with a new
`check-examples.ts` script that runs examples in isolated Vercel
Sandboxes
- Consolidates 5 separate CI jobs into a single `check-examples` job
that runs on every PR
- Removes ~30 test package directories and associated helper scripts
Now, checking **all examples** takes ~2:30. Before even checking one of
them took that long.
## Testing notes
The new checker validates all examples marked with
`maintainedByCoreTeam: true` in their `meta.json`. It installs
dependencies, runs all non-persistent `turbo` tasks, and verifies cache
hits on second run.
