Add pre-refresh-pacman.d hooks for custom repository support (#5681)

Fires after the channel template is copied to /etc/pacman.conf and before
`pacman -Syyuu` runs, letting users layer custom Pacman repositories or
IgnorePkg directives onto the freshly-written config so they're respected
by the upgrade. Useful for users running on overlay distros (CachyOS,
Chaotic-AUR, internal company repos) where a wipe-and-reinstall would
otherwise pull vanilla Arch versions.

Ships with `pre-refresh-pacman.d/add-custom-repo.sample` showing the
pattern.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

bin/omarchy-refresh-pacman

@@ -19,5 +19,8 @@ echo
 sudo cp -f "$OMARCHY_PATH/default/pacman/pacman-$channel.conf" /etc/pacman.conf
 sudo cp -f "$OMARCHY_PATH/default/pacman/mirrorlist-$channel" /etc/pacman.d/mirrorlist
 
+# Allow user customization of /etc/pacman.conf before the upgrade runs
+omarchy-hook pre-refresh-pacman
+
 # Reset all package DBs and then update
 sudo pacman -Syyuu --noconfirm