macOS runner clock is NTP-synced before the job starts, so step and metric timelines match

Runner-job metric charts on macOS jobs now line up with the step timeline. A freshly booted macOS VM could be handed a job before its guest clock had synced, so GitHub step timestamps drifted minutes off the actual run. The new runner image forces a synchronous NTP step (sntp -sS) right after dispatch and before the runner and metrics sampler start, so steps and metrics share one timeline. Hovering a step now highlights the matching slice of the chart.