addendum 21: DOCFIX-091 tenant onboarding contract (A1 rulings) ...

New docs/tenant-onboarding-contract.md: operator/tenant boundary + rationale,
explicit role table (manager persona, never admin), tenant intake list,
stage dangers, onboarding confirmation set, hardening register H1-H5
(proposals logged, none implemented). v2-DRAFT banner defers to it.

Revert: per addendum-21 REVERT line (doc-only; no cloud state touched).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>