Review notes, kept in <git common dir>/declutter/notes.json until cleared or
posted. A store can be scoped to one review; it then reads and writes only that
review’s notes, and leaves the others alone.
Which changes the reviewer has marked as reviewed. A mark belongs to a file’s exact
change — its path and both versions — so it lapses as soon as either version moves,
and it carries over between ranges that contain the same change.