Add Omarchy 4 upgrade bridge

Introduce a self-contained omarchy-upgrade-to-4 command for legacy installs and remove the 4.0 package-layout transition from the normal migration runner.

migrations/1780000000_4_0.sh

@@ -1,24 +0,0 @@
-echo "Migrate to Omarchy 4.0 package layout"
-
-# Omarchy 4.0 collapses the old git/online-installer layout into the packaged
-# runtime layout. Packages own most static system files; these commands refresh
-# the machine-specific and user-specific pieces in place.
-channel=$(omarchy-version-channel 2>/dev/null | awk '{print $1}')
-case "$channel" in
-  stable|rc|edge) ;;
-  *) channel=stable ;;
-esac
-
-# Fold post-4.0 pre-release migration work into the single 4.0 migration.
-rm -f "$HOME/.local/bin/playwright-cli"
-omarchy-pkg-drop claude-code github-cli
-
-sudo env \
-  OMARCHY_PATH="${OMARCHY_PATH:-/usr/share/omarchy}" \
-  OMARCHY_INSTALL="${OMARCHY_INSTALL:-${OMARCHY_PATH:-/usr/share/omarchy}/install}" \
-  OMARCHY_INSTALL_USER="$USER" \
-  OMARCHY_MIRROR="${OMARCHY_MIRROR:-$channel}" \
-  /usr/bin/omarchy-setup-system --install-user "$USER" --upgrade
-
-OMARCHY_SETUP_CONTEXT=runtime omarchy-setup-user --force
-omarchy-refresh-applications