feat(sbx): VM dev-toolchain bootstrap + save/load parity across backends

Reorganize the help menu into one VIRTUAL MACHINES cluster covering all
backends, and bring docker/multipass/vbox to save+load parity:

- Launch-time dev toolchain: sandbox-bootstrap.sh + editable
  sandbox-tools.json (vim/curl guaranteed), installed in docker AND
  multipass sandboxes at provision time.
- Vbox load: vm_restore + `/sbx vmload <vm> [label]` (restore snapshot
  then boot the GUI).
- Multipass load: `/sbx load` is now backend-aware (locate_snapshot +
  SnapKind), mp_restore re-attaches the shared shell; teardown stops
  (not purges) an instance that still has snapshots so they survive
  `/sbx stop`.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

hh/scripts/sandbox-bootstrap.sh

@@ -0,0 +1,47 @@
+#!/usr/bin/env bash
+# sandbox-bootstrap.sh — install a baseline dev toolchain inside a hack-house
+# Docker sandbox.
+#
+# It is piped to `bash -s` (as root) inside the container at provision time, so
+# fresh sandboxes come up usable instead of bare. Idempotent + sentinel-guarded:
+# a re-provision (new member joins) or a snapshot load is a fast no-op.
+#
+# When launched by hh, the package list comes from scripts/sandbox-tools.json
+# (the canonical, editable schema) via $HH_SBX_PKGS. DEFAULT_PKGS below is only
+# the fallback for running this script standalone. Per-launch override:
+#   HH_SBX_PKGS="vim tmux ripgrep" ./host-house.sh ...
+set -uo pipefail
+
+SENTINEL=/var/lib/hh-bootstrap.done
+[[ -f "$SENTINEL" ]] && exit 0      # this container is already provisioned
+
+# Baseline dev tools: editors, fetchers, vcs, net + inspect utilities, json,
+# archives. Keep names valid for the base image (ubuntu:24.04) — an unknown
+# package would otherwise abort the whole batch install.
+DEFAULT_PKGS="vim nano less curl wget ca-certificates git \
+build-essential pkg-config \
+procps iproute2 iputils-ping net-tools \
+jq unzip zip tree htop file ripgrep \
+python3 python3-pip python3-venv"
+
+PKGS="${HH_SBX_PKGS:-$DEFAULT_PKGS}"
+
+export DEBIAN_FRONTEND=noninteractive
+
+# Refresh the index first (base images ship without /var/lib/apt/lists). If the
+# update fails (e.g. no network) we bail WITHOUT writing the sentinel, so the
+# next launch retries instead of leaving a half-provisioned shell.
+apt-get update -qq || exit 0
+
+# --no-install-recommends keeps the image lean. apt is atomic on resolution, so
+# if one name is unavailable the batch aborts — fall back to one-by-one so a
+# single bad/missing package can't deprive the shell of everything else.
+# shellcheck disable=SC2086
+if ! apt-get install -y --no-install-recommends $PKGS; then
+    for p in $PKGS; do
+        apt-get install -y --no-install-recommends "$p" || true
+    done
+fi
+
+mkdir -p "$(dirname "$SENTINEL")"
+date -u +%FT%TZ > "$SENTINEL"       # mark done; skip on the next provision pass