Folders without git: each card's changes on Linux

When several agents work in a plain folder at once, Obeya tells each card's changes apart by the time the file system stamps on them. On Linux that stamp comes from a coarse clock that runs slightly behind, so changes made right after an agent's step began were credited to no card, and undo would not have taken them back. Obeya now reads the start of each step off the file system's own clock, so on Linux every card again lists exactly what it changed.