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-refresh-pacman

@@ -23,4 +23,4 @@ sudo cp -f "$OMARCHY_PATH/default/pacman/mirrorlist-$channel" /etc/pacman.d/mirr
 omarchy-hook pre-refresh-pacman
 
 # Reset all package DBs and then update
-sudo env OMARCHY_UPDATE_PACMAN=1 pacman -Syyuu --noconfirm
+omarchy-update-pacman -Syyuu --noconfirm