Update: refuse pre-channel downgrades; doc docker restore scope

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QKxGW1raAWaqeU8WdHsMsp

bin/omarchy-cn-update

@@ -52,6 +52,14 @@ stable)
   ;;
 esac
 
+# Never move onto a tree that predates the channel mechanism: its update
+# command cannot switch back, stranding the install
+if [[ ! -f $src/bin/omarchy-cn-channel ]]; then
+  git -C "$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
+  exit 1
+fi
+
 new=$(git -C "$src" rev-parse --short HEAD)
 echo "Channel: $channel, source: $old -> $new"