feat(sbx): create fresh VirtualBox VMs via cloud-init (/sbx launch vbox new)

The existing vbox paths only open pre-made images; there was no way to
build one from scratch. Add `/sbx launch vbox new [name]` backed by
scripts/vbox-new.sh: download an Ubuntu cloud image, convert it to a VDI,
build a cloud-init NoCloud seed that creates a sudo login user and installs
the sandbox-tools.json toolchain on first boot, then create + boot the VM.

VirtualBox guests can't be exec'd into like docker/multipass, so cloud-init
is the provisioning channel. Generates a one-time login password (or takes
--pass) and authorizes a local SSH key if present. Runs off-thread; rolls
back a half-built VM on failure.

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

hh/src/ui.rs

@@ -165,6 +165,10 @@ fn help_clusters(theme: &Theme) -> Vec<HelpCluster> {
                     "/sbx launch vbox",
                     "VirtualBox VM picker — local GUI (↑↓ move · Enter/Tab boot · Esc dismiss)",
                 ),
+                kv(
+                    "/sbx launch vbox new [name]",
+                    "build a FRESH Ubuntu VM from a cloud image (cloud-init installs the dev toolchain)",
+                ),
                 kv(
                     "/sbx launch vbox [gui] <vm> [yes]",
                     "boot a VirtualBox VM's GUI on YOUR machine (non-host appends yes to install/import first)",