feat(pfi-gx10): register the ASUS Ascent GX10 and convert it to headless

NVIDIA GB10, aarch64, 121 GB unified, sm_121. Ships booting to graphical.target
with GDM and GNOME Remote Desktop running.

playbooks/gx10-headless.yaml sets multi-user.target, stops gnome-remote-desktop,
masks the sleep/suspend/hibernate targets, makes logind ignore lid and idle, and
adds sshd keepalives so a stalled link does not kill a long-running job.

Two things learned the hard way and recorded in the playbook:

- gdm is a STATIC unit on Ubuntu, pulled in by display-manager.service and never
  'enabled'. A guard of  always skips, and a verify
  written the same way passes while the desktop is still running. Both now test
  is-active. The first run reported six green verifies having not stopped gdm.
- elway's --sudo applies only to ad-hoc --shell/--upload. Playbook steps run as
  the connecting user and must carry their own sudo; connect as infra-ops.

The playbook refuses to stop the display manager while a seat session is held,
overridable with --var force_dm_stop=true. Networking is deliberately out of
scope: the box is on a desk on Wi-Fi with a temporary DHCP lease and no ethernet
carrier, and belongs to the rack-install change.

servers/pfi-gx10/ssh-target

@@ -0,0 +1 @@
+infra-ops@10.100.10.226