feat(hack): add scripts to drive the dev environment headless

Start QEMU in the background with a QMP socket and console logs, take
screenshots, press keys and wait for serial output, so the graphical boot
menu can be driven without a terminal or a window. boot.sh gets opt-in
knobs for this and behaves as before otherwise.

hack/dev/run/screenshot.sh

@@ -0,0 +1,21 @@
+#!/bin/sh
+# Saves a screenshot of the guest display as a PNG and prints its path, ready to be read as an image.
+# Usage: screenshot.sh [out.png]
+# Requires QEMU 7.1 or newer for PNG screendumps.
+set -e
+
+cd "$(dirname "${0}")/../../.." || exit 1
+
+. "hack/dev/run/lib.sh"
+require_running
+
+OUT="${1:-${SHOT_DIR}/$(date +%H%M%S).png}"
+case "${OUT}" in
+/*) ;;
+*) OUT="$(pwd)/${OUT}" ;;
+esac
+mkdir -p "$(dirname "${OUT}")"
+
+./hack/dev/run/qmp.sh "{\"execute\":\"screendump\",\"arguments\":{\"filename\":\"${OUT}\",\"format\":\"png\"}}" >/dev/null
+[ -s "${OUT}" ] || die "QEMU did not write ${OUT}"
+echo "${OUT}"