docs: 拆分贡献指南为中英双语独立文件

CONTRIBUTING.md（中文）+ CONTRIBUTING_EN.md（英文），互相链接。

CONTRIBUTING_EN.md

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+# Contributing Guide
+
+> 中文版本：[CONTRIBUTING.md](./CONTRIBUTING.md)
+
+Thank you for considering contributing to cursor-byok!
+
+## Prerequisites
+
+| Dependency | Version |
+|------------|---------|
+| Go | >= 1.25 |
+| Node.js | >= 20 |
+| Yarn | 1.x (classic) |
+| [Task](https://taskfile.dev) | >= 3 |
+| [Wails v3 CLI](https://v3alpha.wails.dev) | alpha.74+ |
+
+Additional Linux dependencies: `libgtk-3-dev`, `libwebkit2gtk-4.1-dev` (required by Wails runtime).
+
+## Quick Start
+
+```bash
+# Install frontend dependencies
+cd frontend && yarn install --frozen-lockfile && cd ..
+
+# Start dev mode (hot reload)
+task dev
+
+# Build for current platform
+task build
+```
+
+## Project Structure
+
+```
+├── main.go                 # Entry point
+├── internal/               # Go backend (proxy, forwarding, client management)
+├── frontend/               # Vue 3 + Vite + Tailwind frontend
+│   ├── src/
+│   │   ├── views/          # Pages
+│   │   ├── components/     # Components
+│   │   ├── i18n/           # Internationalization (zh-CN / en-US / ja-JP / ru-RU)
+│   │   └── state/          # Global state
+│   └── plugins/            # Vite plugins (i18n static scanner, etc.)
+├── prompt/                 # Built-in agent prompt templates
+├── proto/                  # Protobuf definitions
+├── build/                  # Build configs & platform Taskfiles
+├── scripts/                # Helper scripts (release, metrics)
+└── Taskfile.yml            # Top-level task orchestration
+```
+
+## Development Guidelines
+
+### Commit Messages
+
+Follow [Conventional Commits](https://www.conventionalcommits.org/):
+
+```
+feat(proxy): support custom upstream timeout
+fix(i18n): add missing Japanese translation keys
+release: 0.0.42
+```
+
+### Code Style
+
+- Go: follow `gofmt` / `go vet`; no additional linter config.
+- Frontend: Vue SFC + Composition API, Tailwind utility-first.
+- New UI strings must be added to ALL locale files (`frontend/src/i18n/locales/`).
+
+### Branching & PRs
+
+1. Create feature branches from `main`: `feat/xxx`, `fix/xxx`.
+2. Keep PRs small and focused — one problem per PR.
+3. Describe motivation and how to test in the PR description.
+
+## Build & Release
+
+```bash
+# Build all platforms (macOS host only)
+task build:all
+
+# Prepare release assets
+task release:prepare
+
+# Publish to GitHub Releases
+task release:github
+```
+
+## License
+
+By contributing, you agree that your contributions will be licensed under the [MIT License](./LICENSE).