OptionalAuthor'agent' when the widget helper claimed an agent command caused this submit (the wire
carried by: 'agent'); absent means the user. A CLAIM from widget-side code: the board
applies its own agent-window gate before trusting it.
The submitted payload, JSON-serialized (capped at WHITEBOARD_WIDGET_SUBMIT_MAX_CHARS).
The submitting widget's board item ID (e.g. html-2).
The widget's header title ('' when untitled — consumers fall back to ItemID).
One accepted widget submission, surfaced by the board/host
WidgetSubmittedoutput.