fix: suppress exit code 143 from metric logger cleanup

The metric logger is killed via SIGTERM on benchmark completion, producing
exit code 143 (128+15) which propagated through set -e. Added explicit
return 0 / trailing true to cleanup traps.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

scripts/benchmark/run-baseline.sh

@@ -148,6 +148,7 @@ log_info "Metric logger started (PID: $METRICS_PID)"
 cleanup() {
     kill "$METRICS_PID" 2>/dev/null || true
     wait "$METRICS_PID" 2>/dev/null || true
+    return 0
 }
 trap cleanup EXIT