The caller's existing organization, if any

Answers whether a signed-in caller whose session carries no organization already belongs to one — the state left behind when a membership is created after the session was issued, since session claims are only set at authentication. Returns the organization the session should be re-scoped into, or null when the caller has none and creating one is the right next step. Read-only: it never creates or changes anything, and pending invitations do not count as an organization.