trello.addBoardMember adds an existing member to one board.
automations/add-trello-board-member.automation.ts
board as an ID, short link, or standard board URL and a stable memberId. role accepts "admin", "normal", or "observer" and defaults to "normal". Optional allowBillableGuest permits a Workspace administrator to add a multi-board guest. Optional second argument { account } selects a Trello binding.
Returns Signal<{ boardId: string; memberId: string; role: "admin" | "normal" | "observer" }>.