Reject invalid legacy release selectors

scripts/release

@@ -1,10 +1,16 @@
 #!/bin/sh
 set -eu
 
-if [ "$#" -ne 2 ]; then
+usage() {
     printf '%s\n' 'usage: ./scripts/release MODULE VERSION' >&2
     exit 2
-fi
+}
+
+[ "$#" -eq 2 ] || usage
+case "$1" in
+    core|ckconfig|ckprompt) ;;
+    *) usage ;;
+esac
 
 script_dir=$(CDPATH= cd "$(dirname "$0")" && pwd)
 exec "$script_dir/releasekit" "$1" publish "$2"