feat(sbx): install Docker/Multipass on demand from /sbx launch

Previously, launching a sandbox on a machine without the backend binary
died with a raw "not installed" error and no path forward. Now the
missing backend can be installed in-line, gated on explicit consent, so
a fresh checkout can go from zero to a running sandbox without leaving
the TUI.

Folded into the existing `/sbx launch` verb rather than a new command
(menu is already dense): `/sbx launch docker|multipass [image] install`.
The `install` token opts in; without it the user gets an actionable
error naming the exact retry. The token is filtered out of positional
image parsing so it never shadows a custom image.

The install runs off-thread inside the existing spawn_launch task,
before provisioning, so the TUI never blocks and the *launching guard is
cleared via BrokerMsg on failure. A fresh Docker install also leaves its
daemon up, so launch proceeds straight through.

Scripts (detect-first, never silent, --plan dry-run, idempotent if
already present):
- ensure-multipass.sh (new): Linux→snap (clear failure if snapd absent),
  macOS→brew cask, Windows→winget.
- ensure-docker.sh: new --install mode using Docker's OFFICIAL,
  GPG-verified apt repo (docker-ce) on Debian/Ubuntu, with dnf
  (Fedora/RHEL) and pacman (Arch) fallbacks. Deliberately avoids piping
  get.docker.com into a root shell. Existing daemon-start path intact.

sbx.rs: docker_installed()/multipass_installed() detectors and
ensure_docker_install()/ensure_multipass_install() wrappers; ENSURE_MULTIPASS const.

ui.rs: help text documents the [install] option on both backends.

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

hh/src/ui.rs

@@ -154,12 +154,12 @@ fn help_clusters(theme: &Theme) -> Vec<HelpCluster> {
             items: vec![
                 // ── launch (one verb per backend) ──
                 kv(
-                    "/sbx launch docker [image]",
-                    "Linux container — shared shell relayed to the room (default ubuntu:24.04 + auto dev toolchain)",
+                    "/sbx launch docker [image] [install]",
+                    "Linux container — shared shell relayed to the room (default ubuntu:24.04 + auto dev toolchain; append install if Docker is missing)",
                 ),
                 kv(
-                    "/sbx launch multipass [image]",
-                    "full Ubuntu VM — shared shell relayed to the room (same dev toolchain)",
+                    "/sbx launch multipass [image] [install]",
+                    "full Ubuntu VM — shared shell relayed to the room (same dev toolchain; append install if Multipass is missing)",
                 ),
                 kv(
                     "/sbx launch vbox",