fix(lgtm-datamesh): walkthrough preflight probes OpenMetadata in wrong namespace

ready_by_label takes <namespace> <label-selector>, but the lineage-act
preflight passed 'openmetadata' as the namespace. OpenMetadata lives in
the capstone namespace, so the probe queried a namespace that doesn't
exist and reported 'OpenMetadata server Ready ✗' even with the server
Running 1/1 — blocking the walkthrough on a healthy cluster.

Verified live: preflight for --only lineage now passes and the act runs.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: patterncatalyst

examples/lgtm-datamesh/demos/walkthrough.sh

@@ -233,7 +233,7 @@ preflight() {
     fi
     if want_act lineage; then
         check "OpenMetadata server Ready" \
-              "ready_by_label openmetadata 'app.kubernetes.io/name=openmetadata'" \
+              "ready_by_label $NS 'app.kubernetes.io/name=openmetadata'" \
               "./scripts/setup-openmetadata.sh"
         check "OpenMetadata ingestion has run (catalog populated)" \
               "kubectl get job -n $NS om-ingest-postgres om-ingest-kafka om-declare-lineage --no-headers 2>/dev/null | wc -l | grep -q '^3$'" \