The watched-trip updates log filtered to entries AFTER `since`.
The notification payload: everything logged for the trip strictly after
the `since` ISO-8601 timestamp (the user's last-seen time), oldest-first.
Returns {trip_id, since, new_update_count, updates}. Use this tool to
fetch the stored update entries the user has not seen yet; for a live
recomputed diff, use whats_changed instead. An empty `since` returns the whole
log. Id is regex-validated (^trip-[a-z0-9-]+$); unknown id ->
{"error": "not_found"}, malformed id -> {"error": "invalid_trip_id"}.
Connector