Shield Omarchy pacman transactions from desktop session teardown

Upgrading systemd runs its post_upgrade scriptlet mid-transaction, which
reexecs both the system manager and every user manager. When pacman runs
inside a user-session scope (the floating update terminal), that reexec
can SIGKILL it and abandon the transaction halfway, with packages
upgraded but none of the post-transaction hooks run.

Route every Omarchy-owned system mutation through a new hidden
omarchy-update-pacman helper that registers the transaction as a PID 1
scope via systemd-run, keeping it out of the user manager's cgroups.
System scopes survive the system manager's own reexec, and as a bonus the
transaction now also survives its terminal window closing. On unbooted
systems (the installer chroot) the helper runs pacman directly.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

bin/omarchy-update-system-pkgs

@@ -10,7 +10,7 @@ set -e
 # a stream, because an upgrade without --noconfirm prompts on stderr. The
 # handler has already said why, so no heading here either.
 if [[ ${OMARCHY_UPDATE_CONFLICT:-} == 1 && ${OMARCHY_UPDATE_INTERACTIVE:-} == 1 ]]; then
-  exec sudo env OMARCHY_UPDATE_PACMAN=1 pacman -Syu --overwrite '/usr/share/omarchy/*'
+  exec omarchy-update-pacman -Syu --overwrite '/usr/share/omarchy/*'
 fi
 
 echo -e "\e[32m\nUpdate system packages\e[0m"
@@ -23,7 +23,7 @@ trap 'rm -f "$errors"' EXIT
 #
 # Progress bars stay on stdout. Errors are on stderr, kept for the conflict
 # handler below; LC_ALL=C is what keeps them parseable in any locale.
-if sudo env LC_ALL=C OMARCHY_UPDATE_PACMAN=1 pacman -Syu --noconfirm \
+if LC_ALL=C omarchy-update-pacman -Syu --noconfirm \
   --overwrite '/usr/share/omarchy/*' 2>"$errors"; then
   cat "$errors" >&2
   exit 0