Pin deployment to amd64 nodes

The image is built for linux/amd64, but without a nodeSelector the pod
scheduled onto the ARM64 node (deg-pi) and died with 'exec format
error'. Select nodes by the architecture label instead of a hostname.

k8s-deploy.yaml

@@ -38,6 +38,10 @@ spec:
     spec:
       imagePullSecrets:
         - name: registry-secret
+      # Our image is built for linux/amd64 only; pin to x86 nodes so the pod
+      # never lands on ARM nodes (e.g. deg-pi) with an exec format error.
+      nodeSelector:
+        kubernetes.io/arch: amd64
       containers:
         - name: woodpecker-mcp
           # Immutable per-commit tag (rendered by envsubst in CI from CI_COMMIT_SHA).