feat(sandbox): VM library — pointer catalog + local build via /sbx vmlib

Add an installable-VM catalog that ships pointers only (no multi-GB images
in the repo). When a VM is chosen it is BUILT LOCALLY on the caller's own
machine — nothing is relayed to the room.

- scripts/vbox-library.json: 7-entry manifest (Windows 11, macOS Sonoma,
  Kali, Parrot, Ubuntu 24.04, Fedora 41, Debian 12) with download
  pointers, ostype, cpu/mem/disk, and build kind.
- scripts/vbox-library.sh: --list / --info / --plan / --install. Build
  kinds: iso (download/--iso + createvm + boot installer; EFI+TPM for
  Win11), cloudimg (delegate to vbox-new.sh, unattended), ova (import),
  manual (pointer-only, e.g. macOS per Apple licensing). Detect-first:
  --plan changes nothing, install refuses to clobber and rolls back
  half-built VMs, direct URLs fall back to the page + --iso <path>.
- sbx.rs: vbox_library()/library_vm()/vbox_library_install() loaders +
  running_vms() for live-state markers.
- app.rs: /sbx vmlib (catalog ✓installed/↓available), /sbx vmlib <id>
  (pointer/notes), /sbx vmlib <id> install [--iso path] (local build);
  /sbx vms now flags running VMs (▶). Registered in SBX_SUBCOMMANDS +
  usage.
- ui.rs: help entry for the VM library.

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

hh/src/ui.rs

@@ -293,6 +293,10 @@ fn help_clusters(theme: &Theme) -> Vec<HelpCluster> {
                     "/sbx vbox new [name]",
                     "build a FRESH Ubuntu VM from a cloud image (cloud-init installs the dev toolchain)",
                 ),
+                kv(
+                    "/sbx vmlib [<id> [install]]",
+                    "VM library — catalog of installable VMs (Win11, macOS, Kali…); <id> install builds it LOCALLY on your machine (pointers only, no images bundled)",
+                ),
                 kv(
                     "/sbx vbox [gui] <vm> [yes]",
                     "boot a VirtualBox VM's GUI on YOUR machine (non-host appends yes to install/import first)",