Cloud sessions can be created through a newer request shape

Creating a cloud session can now build a different request body for the newer session-create endpoint, carrying worker, source, outcome, seed bundle and branch-reuse fields in place of the older single session-context blob. It applies only when an account-level check passes and the session comes from ultrareview or ultraplan; nothing in this build sets that check, so what accounts get is decided elsewhere. --project and agent dispatch still refuse outright on accounts still using the old endpoint.