Phase 2 of the multi-account segregation plan: codifies the OU tree
(workloads/prod/nonprod, security, sandbox, graveyard) and three
org-wide SCPs (workloads-region-lock, protect-security-baseline,
deny-root-user) generalized from the proven external-dev guardrails.
All resources Retain — CFN must never detach a live guardrail. New SCPs
attach only to the new empty OUs; extending to external-dev is a
separate gated targetIds change after live verification.
