docs: keep only bilingual (English + Simplified Chinese) README and CONTRIBUTING

Remove ja/ko/ru/zh-TW translations of README.md and CONTRIBUTING.md.
The tracker workflow now only syncs docs/README.zh-CN.md; language links
in README.md, CONTRIBUTING.md, and the AGENTS.md layout are updated to
the two maintained languages.

Co-Authored-By: Claude <noreply@anthropic.com>

AGENTS.md

@@ -28,7 +28,7 @@ tools/              # Go CLI tooling (modular, zero dependencies)
 preview/            # Browser-based live preview
   index.html        # SPA with style/template/client/viewport switching
   rendered.js       # Pre-rendered HTML (generated, committed for clone-and-preview)
-docs/               # Multi-language documentation
+docs/               # Bilingual documentation (English + Simplified Chinese only)
 ```
 
 ## Working With Templates