Adopt scripts-to-rule-them-all: script/ entrypoints, Makefile shims

script/fmt

@@ -0,0 +1,14 @@
+#!/bin/sh
+# script/fmt: format all files (writes).
+set -eu
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+
+main() {
+    cd "$ROOT"
+    echo "Formatting code..."
+    # shellcheck disable=SC2046 # word splitting of file list is wanted
+    gofmt -w $(find . -name '*.go' -not -path './vendor/*')
+}
+
+main "$@"