Make install.sh mode-aware with OMARCHY_INSTALL_MODE

Three modes the installer now branches on:

  iso-chroot      Running inside the new system's chroot from the ISO build.
                  pacman is configured by archinstall; systemd isn't running,
                  so service enables skip --now.
  online-package  Running on an existing Arch where the omarchy-* packages
                  are already installed. Skip base.sh and the bootstrap
                  preflight steps; only the per-host runtime ops in
                  config/login/post-install need to run.
  online-git      boot.sh path: clone into $HOME/.local/share/omarchy and
                  bootstrap everything from scratch.

install.sh derives OMARCHY_PATH from its own location, so the same script
works whether it's at /usr/share/omarchy/install.sh (package mode) or
$HOME/.local/share/omarchy/install.sh (git mode). install_mode_is helper
replaces direct ${OMARCHY_CHROOT_INSTALL:-} / ${OMARCHY_ONLINE_INSTALL:-}
checks in chroot.sh, finished.sh, errors.sh, mise-work.sh, preflight/pacman.sh.

Legacy vars still work through export_legacy_mode_flags, which sets
OMARCHY_CHROOT_INSTALL=1 / OMARCHY_ONLINE_INSTALL=true based on the
canonical mode for any caller that hasn't been updated.

bin/omarchy-install-mode prints the current mode for scripts/users that
need to introspect.

Smoke-tested all four entry paths (explicit mode, both legacy shims,
auto-detection from OMARCHY_PATH); each resolves correctly and the
legacy flags are exported in matching modes.

install/post-install/finished.sh

@@ -29,7 +29,7 @@ if gum confirm --padding "0 0 0 $((PADDING_LEFT + 32))" --show-help=false --defa
   # Clear screen to hide any shutdown messages
   clear
 
-  if [[ -n ${OMARCHY_CHROOT_INSTALL:-} ]]; then
+  if install_mode_is iso-chroot; then
     touch /var/tmp/omarchy-install-completed
     exit 0
   else