Move the cn migration call into omarchy-migrate and guard downgrades onto pre-migrate trees

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Kb7vDj6PHwymeDeSUUk1eX

bin/omarchy-cn-update

@@ -54,11 +54,11 @@ stable)
   ;;
 esac
 
-# Never move onto a tree that predates the channel mechanism: its update
-# command cannot switch back, stranding the install
-if [[ ! -f $src/bin/omarchy-cn-channel ]]; then
+# Never move onto a tree that predates the channel or migrate mechanism:
+# it cannot switch back or finish this update's migration step
+if [[ ! -f $src/bin/omarchy-cn-channel || ! -f $src/bin/omarchy-cn-migrate ]]; then
   git -C "$src" checkout -q "$old"
-  echo "$channel 通道的目标发布早于通道机制，已回退；请使用 nightly 或更新的发布" >&2
+  echo "$channel 通道的目标发布早于当前更新机制，已回退；请使用 nightly 或更新的发布" >&2
   exit 1
 fi