Adds 'openshell doctor check' that validates Docker connectivity before
gateway setup. Reports pass/fail with version info and DOCKER_HOST
status, giving users a quick way to verify their environment.
* feat(sandbox): add gpu sandbox scheduling support
Allow sandbox creation to request GPU resources explicitly or infer them from GPU image names. This wires GPU intent through bootstrap, validates gateway support, and adds dedicated GPU E2E coverage for follow-up cluster testing.