Make script/ entrypoints POSIX sh; add bootstrap and setup

script/precommit

@@ -1,9 +1,9 @@
-#!/usr/bin/env bash
+#!/bin/sh
 # script/precommit: run by the git pre-commit hook; fails the commit if
 # checks fail. Our own extension to scripts-to-rule-them-all.
-set -euo pipefail
+set -eu
 
-SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd -P)"
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
 
 main() {
     "$SCRIPT_DIR/check"