The one-line reminder shown on a prospect's card — "coffee Thursday",
"waiting on their broker".

Deliberately not part of the card move. Moving a card is a status
transition and is recorded in the activity trail; jotting a note is not,
and writes nothing to it. Sending both through one endpoint would either
fill the trail with note edits or make a move's meaning depend on which
fields happened to be present.

Send an empty string to clear it — the timestamp clears with it, so a
stale "updated 3 weeks ago" is never left attached to a note that is
gone.