docs: Add Chinese (zh-CN) translation

## Motivation

gstack is a great tool for Claude Code workflows. Adding Chinese documentation would help Chinese-speaking developers understand and use this project more easily.

## Proposed Approach

I propose adding Chinese translations in a dedicated `docs/zh-CN/` directory:

```
docs/
├── zh-CN/
│ ├── README.md
│ ├── CONTRIBUTING.md
│ ├── ARCHITECTURE.md
│ ├── BROWSER.md
│ └── skills/
│ └── (skill documentation)
```

## Scope

**Phase 1:** Core documentation (README, CONTRIBUTING, skills overview)
**Phase 2:** Technical docs (ARCHITECTURE, BROWSER)
**Phase 3:** All 21 skill documentation files

## Questions for Maintainers

1. Is this something you'd be interested in accepting?
2. Is the proposed directory structure acceptable?
3. Any preferences on how to keep translations in sync with English docs?
4. Should SKILL.md templates also be translated, or keep them English-only?

I'm happy to start with Phase 1 and iterate based on feedback.

---

## 动机 (Chinese)

gstack 是一个很棒的 Claude Code 工作流工具。添加中文文档将帮助中文开发者更容易理解和使用这个项目。

## 建议方案

我建议在 `docs/zh-CN/` 目录下添加中文翻译。

## 范围

- Phase 1: 核心文档 (README, CONTRIBUTING, skills overview)
- Phase 2: 技术文档 (ARCHITECTURE, BROWSER)
- Phase 3: 所有 21 个技能文档

我很乐意从 Phase 1 开始，并根据反馈进行迭代。