Remove CodeSandbox CI integration (#1057)

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.)

Co-authored-by: Erik Rasmussen <erik@mini.local>

Author: erikras-richard[bot]