Update the agent's workflow draft

Autosave endpoint for the canvas. Accepts a partial or full conversation_workflow.v1 draft plus optional layout (node positions/viewport/collapsed groups, never part of the compiled contract). An invalid intermediate state is allowed to save; the response's validation field reports errors/warnings without blocking the save. Send base_revision -- the revision field from the draft you edited -- to have a write refused when someone else has changed the draft since; the 409 body carries the current draft under 'current' so the caller can reconcile without another request. Omitting base_revision keeps last-write-wins.