ci: Add shellcheck for scripts (#20631)

Fixes shellcheck errors in script/*
Adds a couple trailing newlines.
Adds `script/shellcheck-scripts` and associated CI machinery.
Current set ultra-conservative, does not output warnings, only errors.

script/kube-shell

@@ -1,6 +1,6 @@
 #!/bin/bash
 
-if [[ $# < 1 ]]; then
+if [[ $# -ne 1 ]]; then
   echo "Usage: $0 [production|staging|...]"
   exit 1
 fi
@@ -8,4 +8,4 @@ fi
 export ZED_KUBE_NAMESPACE=$1
 
 pod=$(kubectl --namespace=${ZED_KUBE_NAMESPACE} get pods --selector=app=zed --output=jsonpath='{.items[*].metadata.name}')
-exec kubectl --namespace $ZED_KUBE_NAMESPACE exec --tty --stdin $pod -- /bin/bash
+exec kubectl --namespace $ZED_KUBE_NAMESPACE exec --tty --stdin $pod -- /bin/bash