Adds optional REDEPLOY_SUMMARY_JSON path; when set, redeploy-env writes
a structured per-service record array {service,status,error?}. PR #5093's
exit-code contract is preserved (staging=0, prod=1-on-failure).
Consumed by showcase_deploy.yml to fail the workflow on staging per-service
errors without changing the script's exit semantics. Refs spec §3.