Add --headless flag to daemon (no X11 display in container)

GLFW viewer requires X11 DISPLAY, which doesn't exist in Docker.
The --headless flag skips viewer initialization.

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

supervisord.demo.conf

@@ -11,7 +11,7 @@ pidfile=/tmp/supervisord.pid
 childlogdir=/tmp
 
 [program:daemon]
-command=reachy-mini-daemon --sim
+command=reachy-mini-daemon --sim --headless
 autostart=true
 autorestart=true
 stdout_logfile=/dev/stdout