What applying the current context would do to this board

A DRY RUN. The grounding-scoped planner turn runs and nothing is written — not the canvas, not the transcript, not the bake. Returns a job_id; poll /jobs/{id} for the item-level change set plus a review_id naming the candidate held server-side.

A context edit is otherwise silently inert on an existing board until someone re-plans it, and the re-plan then rewrites the board with no way to see first what it would do.

review_id is null when the context changes nothing. That is an outcome, not a failure: the client says so rather than showing an empty list.