Clarify Kanban assistant limits and preserve partial results

The Workspace assistant now treats registered operations as authoritative: it does not offer or confirm an unsupported Kanban board or card write, and a board read cannot substitute for a missing create or edit operation. Kanban board lists use the shared result format, including empty lists, so a saved list remains visible when a later provider request fails. The Kanban Help article describes the available list, search, and summary actions.