trello.onCardMemberAdded matches Trello’s addMemberToCard action.
automations/watch-trello-card-members.automation.ts
board as an ID, short link, or URL. Optional static account defaults to the project binding.
Returns a resource event with actionType: "addMemberToCard", required card, member, and board, plus actionId, occurredAt, optional memberCreator, and the complete provider event.