The Echo-Core scheduler's report_on='changes' contract parses
^GSTACK-CRON: changes=\d+$ from stdout to decide whether to forward
the run's output to a channel. monitor_v2.py now prints that marker
as its final stdout line with num_changes from the current run.
Also switches the success return value from len(all_changes) to 0.
Previously, any run that detected changes (N>0) exited with a non-zero
status, which the scheduler treats as an error (always forwarded,
ignoring report_on). Exit code now signals only fatal errors; the
marker carries the change count.
