Workstream cards keep their own workstream

When a plan doc dropped finished workstreams and added new ones, Obeya could give a new workstream the card of an old one, with its conversation and its done state. Obeya now knows each workstream by its code (W3, S1, 4A …) or by its title, never by its place in the doc. Canvases already mixed up are put right once: a card holding another workstream's history leaves the project under that workstream's title and goes into the archive, and the workstream gets a fresh card.