Make script/ entrypoints POSIX sh; add bootstrap and setup

script/bootstrap

@@ -0,0 +1,65 @@
+#!/bin/sh
+# script/bootstrap: install all dependencies needed to build and develop
+# this repo. Idempotent: every install is guarded by a check so already
+# installed tools are skipped. Installs base tooling with nix, apt, or
+# brew (detected in that order); assumes nothing is present.
+set -eu
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+
+PKGMGR=""
+SUDO=""
+
+detect_pkgmgr() {
+    if command -v nix-env >/dev/null 2>&1; then
+        PKGMGR="nix"
+    elif command -v apt-get >/dev/null 2>&1; then
+        PKGMGR="apt"
+    elif command -v brew >/dev/null 2>&1; then
+        PKGMGR="brew"
+    else
+        echo "bootstrap: no supported package manager (nix, apt, brew)" >&2
+        exit 1
+    fi
+    if [ "$PKGMGR" = "apt" ]; then
+        export DEBIAN_FRONTEND=noninteractive
+        if [ "$(id -u)" != "0" ]; then
+            SUDO="sudo"
+        fi
+    fi
+}
+
+# pkg_install <nix-attr> <apt-pkg> <brew-formula>
+pkg_install() {
+    case "$PKGMGR" in
+        nix) nix-env -iA "nixpkgs.$1" ;;
+        apt) $SUDO env DEBIAN_FRONTEND=noninteractive apt-get install -y "$2" ;;
+        brew) brew install "$3" ;;
+    esac
+}
+
+missing() {
+    ! command -v "$1" >/dev/null 2>&1
+}
+
+main() {
+    cd "$ROOT"
+    detect_pkgmgr
+
+    if missing git; then pkg_install git git git; fi
+    if missing make; then pkg_install gnumake make make; fi
+    if missing node; then pkg_install nodejs nodejs node; fi
+    if missing yarn; then
+        # corepack ships with node >= 16 and provides yarn
+        if command -v corepack >/dev/null 2>&1; then
+            corepack enable
+        else
+            pkg_install yarn yarnpkg yarn
+        fi
+    fi
+
+    yarn install --frozen-lockfile
+    echo "bootstrap complete"
+}
+
+main "$@"