trello.onCustomFieldValueUpdated matches Trello’s updateCustomFieldItem action for card values.
automations/watch-trello-custom-field-values.automation.ts
board as an ID, short link, or URL. Optional static account defaults to the project binding.
Returns a resource event with actionType: "updateCustomFieldItem", required card, customField, customFieldItem, and board, plus actionId, occurredAt, optional old and memberCreator, and the complete provider event.