Onboarding hierarchy: worked-example diagram + ways to split Spaces

Onboarding now teaches the Space → Topic → Memory hierarchy with a real worked example — an "Acme" organization with a team Space, a project Space, and a personal Space — instead of an abstract diagram, and the same example appears both in the app and in the onboarding agent's guide so they teach the same shape.
The onboarding agent can now also suggest concrete ways to split Spaces (by team, product line, audience, client, initiative, or a dedicated onboarding Space) when helping a team lead plan their structure.