feat(anaf): emit GSTACK-CRON marker and exit 0 on successful run

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.

tools/anaf-monitor/monitor_v2.py

@@ -365,8 +365,15 @@ def main():
     
     log("=== Monitor complete ===")
 
+    num_changes = len(all_changes)
     print(json.dumps({"changes": all_changes}, ensure_ascii=False, indent=2))
-    return len(all_changes)
+    # GSTACK-CRON marker: contract with Echo-Core scheduler (report_on="changes").
+    # The shell-kind scheduler parses the last line matching
+    # ^GSTACK-CRON: changes=\d+$ to decide whether to forward stdout.
+    # A successful run (even with N>0 changes) must exit 0 — the scheduler
+    # reports non-zero exit codes as errors unconditionally.
+    print(f"GSTACK-CRON: changes={num_changes}")
+    return 0
 
 if __name__ == "__main__":
     exit(main())