docs: add ADR 013 to warn on Top-Level-Group creation in orgs

What

Add Organizations ADR 013 documenting the decision to warn users when creating a new Top-Level-Group inside an organization, until organization-level billing ships.

Why

Subscriptions (Free, Premium, Ultimate) and credits are scoped per Top-Level-Group. When a user creates a new Top-Level-Group inside an organization, it does not inherit the license or credits from sibling Top-Level-Groups, which can cause confusion. A warning at creation time keeps customers free to structure their organization with multiple Top-Level-Groups while making the per-Top-Level-Group billing boundary explicit. The warning is temporary and is removed once organization-level billing exists.

Assisted-by: Claude:claude-opus-4-7