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/packages.x86_64

@@ -151,6 +151,11 @@ plymouth
 # (rebuild via finish-tanin-repo.sh / the calamares one-liner in BUILD.md).
 cage
 calamares
+# seatd: cage/wlroots needs a seat; the getty-autologin has no logind seat, so
+# run seatd (disabled on the target — greetd uses logind there).
+seatd
+# qt6-wayland: calamares (Qt) needs the wayland platform plugin to run under cage
+qt6-wayland
 
 # resolve provider prompts non-interactively (mkarchiso can't answer them):
 # audio -> pipewire's JACK (matches the pipewire stack); netfilter -> iptables(-nft)