docs: add Chinese (zh-CN) documentation

- Add language switch link to root README
- Translate README.md to docs/zh-CN/README.md
- Translate USER-GUIDE.md to docs/zh-CN/USER-GUIDE.md
- Translate all 13 reference documents to docs/zh-CN/references/

Key terminology mappings:
- context engineering → 上下文工程
- spec-driven development → 规格驱动开发
- context rot → 上下文衰减
- phase → 阶段
- milestone → 里程碑
- roadmap → 路线图

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

@@ -6,6 +6,8 @@
 
 **Solves context rot — the quality degradation that happens as Claude fills its context window.**
 
+[**English**](README.md) | [**简体中文**](docs/zh-CN/README.md)
+
 [![npm version](https://img.shields.io/npm/v/get-shit-done-cc?style=for-the-badge&logo=npm&logoColor=white&color=CB3837)](https://www.npmjs.com/package/get-shit-done-cc)
 [![npm downloads](https://img.shields.io/npm/dm/get-shit-done-cc?style=for-the-badge&logo=npm&logoColor=white&color=CB3837)](https://www.npmjs.com/package/get-shit-done-cc)
 [![Tests](https://img.shields.io/github/actions/workflow/status/glittercowboy/get-shit-done/test.yml?branch=main&style=for-the-badge&logo=github&label=Tests)](https://github.com/glittercowboy/get-shit-done/actions/workflows/test.yml)