iso: fix installer autostart (root uses zsh) + TANINUX branding

- /root/.zprofile launches tanin-installer on tty1 (root shell is zsh, so
  .bash_profile was ignored); striplive removes .zprofile on the target
- profiledef: MERGE releng's file_permissions (lost .automated_script.sh +x ->
  the 'permission denied' message) + keep our entries
- branding: TANINUX os-release/hostname/motd; bootstrap rebrands the boot menus

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

iso/overrides/airootfs/etc/motd

@@ -0,0 +1,5 @@
+
+  TANINUX (camel) — a curated Arch desktop: niri + eww.
+
+  The installer launches on tty1. To start it manually:  tanin-installer
+