square.onGiftCardCustomerLinked subscribes to gift_card.customer_linked for the selected seller. It requires GIFTCARDS_READ.
Example
automations/square-on-gift-card-customer-linked.automation.ts
Event
A customer was linked to a gift card. The signal resolves to the officialSquare.GiftCardCustomerLinkedEvent payload with type narrowed to gift_card.customer_linked.