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>

docs/FOLLOWUPS.md

@@ -249,9 +249,12 @@ TROUBLESHOOTING.md §13.
 
 **Done so far**: the *desktop* piece of the x11 backend is now reproducible.
 `install.sh` detects `capture = x11` (via `capture_backend_is_x11`) and installs
-+ enables `files/headless-desktop.service` (Openbox on `:0`) so the headless
-Xorg has a window manager to render — without it, capture is a black screen.
-`status.sh` gained a matching check (FAIL if no WM on `:0`). Still outstanding:
++ enables `headless-desktop.service` so the headless Xorg has a desktop to
+render — without it, capture is a black screen. Desktop is selectable via
+`HEADLESS_DESKTOP`: `gnome` (default — full Ubuntu GNOME session, X11 path) or
+`openbox` (lightweight bare WM); each ships as its own unit template
+(`files/headless-desktop-{gnome,openbox}.service`). `status.sh` gained a matching
+check (FAIL if no desktop on `:0`, and reports which one). Still outstanding:
 the `--backend x11|wlr` flag/auto-detect, the `config.sh` x11 conf variant, and
 shipping the `xorg-headless.service` + `xorg-headless.conf` templates.