trello.listBoardCustomFields lists the Custom Field definitions available to cards on one board.
automations/list-trello-custom-fields.automation.ts
board as an ID, short link, or standard board URL. Optional second argument { account } selects a Trello binding.
Returns Signal<TrelloCustomField[]>, including each definition’s ID, board model ID, name, type, position, card-front display setting, optional field group, and dropdown options.