#978 Sandbox image missing from the docker host made every shell/code task fail; nothing re-pulls it or reports it

EARS: While the pinned [sandbox] image is absent from a docker host that runs workers, the Highway health endpoint shall report the sandbox as unavailable; the deploy shall pre-pull the image on every docker host. EVIDENCE (#961, 2026-10-20 00:19Z): python:3.11-slim@sha256:e88e9763... absent (docker system df: build cache 0 B, consistent with a prune). basic_hello_world run 38436ed1 failed SandboxUnavailableError (fail-closed as designed). GET /api/v1/health did not flag it. Restored by docker pull of the exact digest; rerun 47308890 completed.