Remove CodeSandbox CI integration

The CodeSandbox CI has been causing build failures due to native
module dependencies (iltorb) that require build tools not available
in the CI environment.

This integration provided minimal value - it built example sandboxes
to verify they still work, but the frequent failures and maintenance
burden outweigh the benefits.

Fixes CI failures seen in recent PRs (#1056, etc.)

Author: Erik Rasmussen