The cluster health check now verifies that /opt/openshell/bin/openshell-sandbox
exists in the gateway container. Without this binary, every sandbox pod crashes
with 'no such file or directory' but the gateway previously reported healthy.
- Add HEALTHCHECK_MISSING_SUPERVISOR marker to cluster-healthcheck.sh
- Add early detection in bootstrap polling loop (runtime.rs)
- Add structured error diagnosis with recovery steps (errors.rs)
- Update debug-openshell-cluster skill with new failure pattern