iso: launch installer via greetd initial_session (proper seat for cage)

seatd alone didn't fix it (getty-autologin gives cage no logind seat; 5x respawn
then systemd start-limit -> black). Switch to greetd's initial_session, which
creates a real logind session WITH A SEAT and auto-runs cage+calamares (no login
prompt). Calamares' striplive restores the tanin-greet greetd config on the target.
Drop the getty-autologin + /root/.zprofile hack.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

iso/build-profile/airootfs/etc/calamares/modules/shellprocess-striplive.conf

@@ -9,3 +9,4 @@ script:
 - "rm -f /root/.zprofile"
 - "rm -f /usr/local/bin/tanin-installer"
 - "systemctl set-default graphical.target"
+- "systemctl disable seatd.service"