A Muir Beach couple is suing the California Coastal Commission, alleging that the agency overstepped its bounds when it ordered them to scale back their planned seaside home. Graham Groneman and his wife, Brett Sibley, have been fighting for several years to build a house, a garage and a storage unit overlooking the Pacific Ocean, next door to Mr. Groneman’s parents. In February, the coastal commission said they could build—but only if they moved the…