feat(registry-cache): add Docker Hub pull-through cache + dind mirror config

Deploy registry:2 as Docker Hub pull-through cache on chemavx-k8 (hostPort 5000,
ClusterIP 10.43.163.56:5000). Configures dind runner to use local mirror via
daemon.json to eliminate Docker Hub rate limit failures in CI/CD.

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

registry-cache/namespace-registry-cache.yaml

@@ -0,0 +1,9 @@
+apiVersion: v1
+kind: Namespace
+metadata:
+  name: registry-cache
+  labels:
+    kubernetes.io/metadata.name: registry-cache
+spec:
+  finalizers:
+  - kubernetes