autoUpdates is now per-env: staging is enforced "disabled" while prod is
"unmanaged" (the drift gate skips it) so prod stays untouched until a later
migration. The gate enforces managed envs and skips unmanaged ones; the
zero-checked floor applies only to managed envs. Regenerates the SSOT JSON.
New CI gate fails when a live Railway service's autoUpdates diverges from the
SSOT (every service must be disabled). Reads Environment.config (autoUpdates is
not on the typed ServiceSource output), fails closed per-env when it verifies
zero services, and skips cleanly on fork PRs that lack a Railway token.