Simplify OMARCHY_INSTALL_MODE to online/offline only

Drop online-git: the new world has a single package-backed install path
that's either online (pacman pulls from omacom-pkgs.org) or offline (ISO
chroot pulls from the bundled mirror). The git-clone-and-run-installer
path is gone — boot.sh becomes a thin bootstrap that installs the
omarchy-installer package and invokes its installer.

- install/helpers/mode.sh: two modes, validation message updated. Default
  fallback is 'online' (the boot.sh path); OMARCHY_CHROOT_INSTALL=1 still
  shims to offline, OMARCHY_ONLINE_INSTALL=true still shims to online.
- install/preflight/all.sh: first-run-mode + disable-mkinitcpio gated on
  offline (offline is the from-scratch chroot path; online runs on an
  existing system).
- install/preflight/pacman.sh: DELETED. boot.sh now handles repo config
  before install.sh runs.
- install/packaging/all.sh: drop base.sh entirely. Both modes have
  base packages installed via omarchy-installer's depends before
  install.sh runs. base.sh remains on disk in case the ISO mirror build
  flow wants to invoke it directly.
- install/helpers/chroot.sh, errors.sh, post-install/finished.sh,
  config/mise-work.sh: rename mode checks to use online/offline.

install/preflight/all.sh

@@ -1,19 +1,12 @@
 source $OMARCHY_INSTALL/preflight/guard.sh
 source $OMARCHY_INSTALL/preflight/begin.sh
 run_logged $OMARCHY_INSTALL/preflight/show-env.sh
-
-# online-git is the only mode that bootstraps pacman config; iso-chroot has
-# archinstall do it; online-package starts with pacman already configured.
-if install_mode_is online-git; then
-  run_logged $OMARCHY_INSTALL/preflight/pacman.sh
-fi
-
 run_logged $OMARCHY_INSTALL/preflight/migrations.sh
 
-# first-run-mode and disable-mkinitcpio are only meaningful when we're
-# bringing a system up from scratch (boot.sh or ISO). online-package mode
-# is bolting Omarchy onto an existing Arch install — neither applies.
-if ! install_mode_is online-package; then
+# Offline (ISO chroot) bootstraps a system from scratch and needs the
+# first-run-mode sudoers shim plus the temporary mkinitcpio hook freeze.
+# Online installs run on an existing system that already has its own users.
+if install_mode_is offline; then
   run_logged $OMARCHY_INSTALL/preflight/first-run-mode.sh
   run_logged $OMARCHY_INSTALL/preflight/disable-mkinitcpio.sh
 fi