Add README.zh-CN.md with a hand-crafted (non-machine) Chinese
translation of the project README, and add a language switcher
link at the top of both README files.
Code blocks, command names, badges, and links are kept identical to
the English source; only prose is translated.
Co-authored-by: yifosheng001 <yifosheng001@ke.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
