Add Kanban mutations to the Workspace assistant

The Workspace assistant now supports bounded Kanban changes: update or delete boards, create, rename, move, and delete columns, create, update, move, delete, and vote on cards, and add or delete card comments. Destructive deletes require explicit confirmation in the current message. Successful writes return durable server-issued results and links, and a provider retry reuses the receipt instead of repeating the change.