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-tools.json

@@ -0,0 +1,28 @@
+{
+  "_comment": "Packages apt-get installs in every hack-house Docker sandbox at launch. Edit `packages` to add dev tools, then relaunch the sandbox. `vim` and `curl` are ALWAYS installed even if absent here. Names must be valid for the base image (ubuntu:24.04); unknown names are skipped, not fatal. Per-launch override: HH_SBX_PKGS=\"vim tmux\".",
+  "packages": [
+    "vim",
+    "curl",
+    "wget",
+    "ca-certificates",
+    "git",
+    "less",
+    "nano",
+    "build-essential",
+    "pkg-config",
+    "procps",
+    "iproute2",
+    "iputils-ping",
+    "net-tools",
+    "jq",
+    "unzip",
+    "zip",
+    "tree",
+    "htop",
+    "file",
+    "ripgrep",
+    "python3",
+    "python3-pip",
+    "python3-venv"
+  ]
+}