George M. Moffett Jr., widely known as a yachtsman and conservationist, signed an agreement last week by which Felix Neck, more than 200 beautiful acres of widely varying terrain extending into Sengekontacket Pond, will be conveyed to the Massachusetts Audubon Society as a wildlife sanctuary. Part of the acreage is transferred immediately, and the balance will be leased to the Audubon Society until the gift is completed.