fix(monitoring): add Promtail to the correct kustomization and fix repo URL

ArgoCD builds infrastructure/base/monitoring/kustomization.yaml directly —
the loki/kustomization.yaml sub-file was never referenced and was ignored.

- Move Promtail HelmChart into the root monitoring kustomization
- Fix lingering grafana-community repo URL to grafana.github.io

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: soohyunme

infrastructure/base/monitoring/kustomization.yaml

@@ -7,8 +7,14 @@ resources:
 
 helmCharts:
   - name: loki
-    repo: https://grafana-community.github.io/helm-charts
+    repo: https://grafana.github.io/helm-charts
     version: 6.55.0
     releaseName: loki
     namespace: monitoring
     valuesFile: loki/values.yaml
+  - name: promtail
+    repo: https://grafana.github.io/helm-charts
+    version: 6.16.6
+    releaseName: promtail
+    namespace: monitoring
+    valuesFile: loki/promtail-values.yaml