AMALFI, Campania — On a terrace clinging to a cliff high above the town, in the Valle dei Mulini, Italy’s brightest lemons are still picked by hand, knee and back. We met Salvatore Aceto, whose family has worked these near-vertical slopes since 1825. Now he is preparing something new for the punishing harvest. Drones. After a first demonstration a couple of weeks ago, he says he is ready to deploy them within the year to carry loads up and down terraces too steep for any machinery. It could be a revolution, he says, not only for growers but for the coast’s many abandoned buildings.