Default the x11 headless desktop to GNOME (Openbox still opt-in)

Bare Openbox rendered only a blank slate root — usable but not the
desktop wanted. Make the X11/NVENC capture path render a full GNOME
session by default, with Openbox available via HEADLESS_DESKTOP=openbox
for minimal/low-power hosts.

- files/headless-desktop-gnome.service: full Ubuntu GNOME session forced
  onto the X11 path (XDG_SESSION_TYPE=x11, no dbus-run-session so it
  shares the systemd user bus). Renamed the Openbox unit to
  headless-desktop-openbox.service.
- lib/headless.sh: HEADLESS_DESKTOP (default gnome) selects the unit
  template + the packages to install (gnome-session/gnome-shell vs
  openbox/xsetroot).
- install.sh: step message + usage document HEADLESS_DESKTOP.
- status.sh: the :0 desktop check now reports which desktop is running
  (reads _NET_WM_NAME off the supporting-wm-check window, e.g.
  "GNOME Shell").
- docs: TROUBLESHOOTING §13 + FOLLOWUPS P3 updated for the GNOME default
  and the openbox toggle.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

install.sh

@@ -59,6 +59,9 @@ Environment overrides:
   HEADLESS_HOSTS     Comma-separated hostnames that default to headless mode.
                      Unset by default; anything not listed defaults to mirror.
                      Override per-invocation with --headless or --mirror.
+  HEADLESS_DESKTOP   Desktop to render on the headless Xorg :0 for the X11/NVENC
+                     capture path (capture = x11): 'gnome' (default) or 'openbox'
+                     (lightweight, for minimal hosts). Ignored on wlr/kms.
 EOF
 }
 
@@ -194,7 +197,7 @@ main() {
     # (this backend is hand-configured; see FOLLOWUPS.md P3). Harmless no-op on
     # the wlr/kms backends, whose capture source renders for itself.
     if capture_backend_is_x11; then
-      step "Installing headless desktop (Openbox on :0 for the X11 capture path)"
+      step "Installing headless desktop (${HEADLESS_DESKTOP:-gnome} on :0 for the X11 capture path)"
       install_headless_desktop
     fi
     # NOTE: the headless prestart drop-in needs the sunshine unit to already