Marin County Parks and Open Space began work on Monday to restore and decommission over 4,000 linear feet of unsanctioned trails in San Geronimo Valley’s Gary Giacomini Preserve, the largest preserve in West Marin at 1,500 acres. The project figures into the county’s Road and Trail Management Plan, which aims to designate a large swath of open space trails for hiker, biker and equestrian use and has sparked the ire of some residents who feel…