feat(showcase): wire verify-deploy to staging via SSOT, gate on redeploy errors

showcase_deploy.yml is now SSOT-driven: env id and service set both come
from railway-envs.generated.json (no inline matrix, no hardcoded
RAILWAY_ENV_ID). Probes staging on workflow_run (was incorrectly probing
prod). Reads redeploy-env's per-service JSON summary uploaded as an
artifact and fails the workflow on any staging status:error while
verify still runs against the success-set. Preserves PR CopilotKit#5093's
redeploy-env.ts exit semantics unchanged. Refs spec §3.

Author: jpr5