An eerie quiet has descended over the past week on Fulton Street, a stretch of weather-beaten apartment buildings in eastern Linda Vista that is home to a large number of recent immigrants, some of them legal residents, others not. Some people have skipped work. Others have skipped doctor appointments. Children are walking to school more frequently by themselves, neighbors say, because their parents are afraid to accompany them outside. “Today the migra was here,†explained Jose Cardenas, 30, one of those here legally, on a gray afternoon this week when the sidewalks were empty and the street deserted save for...