Emily Rooney has left the Siberian isolation where ABC News closeted her after she lost her job as executive producer of "World News Tonight with Peter Jennings." She has signed with Fox Broadcasting to be senior producer of "On Assignment," of whichformer CBS News Producer David Corvo is executive producer. "On Assignment" this fall replaces Fox's ill-fated "Front Page." There have been suggestions by Fox that "On Assignment" will follow its Sunday NFL telecasts and air opposite CBS' "60 Minutes," where Ms. Rooney's father, Andy Rooney, has long toiled.