Add Chinese translation of README

[VictorVVedtion]

Description

selfmodel has a global audience but the README is English-only. Create a Chinese translation at README.zh-CN.md.

What to do

1. Create README.zh-CN.md with the full README content translated to Simplified Chinese
2. Add a language switcher link at the top of both READMEs:
   • English README: [中文](README.zh-CN.md)
   • Chinese README: [English](README.md)

Notes

• The CLAUDE.md system instructions are already in English by design (for LLM compliance). This is user-facing documentation only.
• Keep code blocks, command examples, and technical terms in English
• Translate headings, descriptions, and explanatory text

Acceptance criteria

• README.zh-CN.md exists with complete translation
• Both READMEs have language switcher links
• Code blocks are preserved as-is