docs(readme): mark translations as pre-v2 pending update

The Chinese and Japanese READMEs still describe the v1 architecture
(setup flow, channel list, "main channel" concept, etc.). Add a notice
at the top of each pointing readers to the English README for current
content until the translations are refreshed.

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

README_ja.md

@@ -14,6 +14,8 @@
   <a href="repo-tokens"><img src="repo-tokens/badge.svg" alt="34.9k tokens, 17% of context window" valign="middle"></a>
 </p>
 
+> **注意:** この日本語訳は v1 時点のもので、最新の v2 アーキテクチャは反映されていません。最新の内容は [README.md](README.md) をご覧ください。
+
 ---
 
 <h2 align="center">🐳 Dockerサンドボックスで動作</h2>