feat(sandbox): podman backend + goose AI harness + noVNC GUI sandbox

Add Podman as a rootless/daemonless sandbox backend alongside Docker,
Multipass and Local, and wire Goose in as the default agentic harness
for the granted `!task` path (bridge execs `<engine> exec <name> goose
run` and streams output to chat; auto-degrades to the simple one-shot
injector when goose is absent).

Add an optional GUI sandbox track (XFCE + TigerVNC + websockify/noVNC on
:6080) summoned via `/sbx <engine> gui`, plus container-side provisioning
in sandbox-bootstrap.sh and a host-side ensure-podman.sh prereq helper.

Refresh the in-app command help to the backend-led `/sbx <engine> [gui]`
grammar and minor ui tweaks.

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

hh/scripts/bootstrap.sh

@@ -49,7 +49,7 @@ for bin in python3 cargo; do
     if have "$bin"; then echo "  ✓ $bin ($($bin --version 2>&1 | head -1))"
     else echo "  ✖ $bin — REQUIRED"; missing=1; fi
 done
-for bin in tmux docker multipass direnv; do
+for bin in tmux docker podman multipass direnv goose; do
     if have "$bin"; then echo "  ✓ $bin (optional)"
     else echo "  · $bin not found (optional)"; fi
 done