Creating a workstream no longer blocks the modal on the whole checkout. The server registers the workstream with silverwood's new `new basic --checkout-extent skip` (fast, validating) and responds at the accept boundary, so `POST /api/sessions` returns as soon as the create is accepted and the modal closes. The checkout is then provisioned in the background via `silverwood workstream <ID> basic checkout`; the node appears pending -> ready/failed on the canvas via the reconcile loop, and the initial terminal spawns once the checkout is ready. This preserves the sync/async distinction: a synchronous validation/precheck error still fails the register step, so the modal stays open with its inline error; an asynchronous provisioning failure surfaces as `active - basic.failed` on the canvas instead. The client modal itself is unchanged (its await-close-on-success / stay-open-on-error is already correct against the fast endpoint); only the workstream-state indicator gets an amber color for the transient `initialized-without-checkout` state. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>