docs: add CONTRIBUTING.md with setup and PR guidelines

CONTRIBUTING.md

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+# Contributing to Agent Browser
+
+Thank you for your interest in contributing to Agent Browser! This document provides guidelines for setting up the development environment, coding style, and submitting pull requests.
+
+## Development Setup
+
+### Prerequisites
+
+- Node.js 18+ 
+- npm or pnpm
+- Git
+
+### Installation
+
+1. Fork the repository
+2. Clone your fork:
+   ```bash
+   git clone https://github.com/YOUR_USERNAME/agent-browser.git
+   cd agent-browser
+   ```
+
+3. Install dependencies:
+   ```bash
+   npm install
+   ```
+
+4. Start development:
+   ```bash
+   npm run dev
+   ```
+
+### Available Scripts
+
+| Command | Description |
+|---------|-------------|
+| `npm run dev` | Start development server |
+| `npm run build` | Build for production |
+| `npm run test` | Run tests |
+| `npm run lint` | Run ESLint |
+| `npm run format` | Format code with Prettier |
+
+## Coding Style
+
+- Follow the existing code style
+- Use TypeScript for all new code
+- Run `npm run lint` before committing
+- Run `npm run format` to ensure consistent formatting
+- Write meaningful commit messages
+
+## Submitting a Pull Request
+
+1. **Create a branch** from `main`:
+   ```bash
+   git checkout -b feature/your-feature-name
+   ```
+
+2. **Make your changes** and commit:
+   ```bash
+   git add .
+   git commit -m "feat: add your feature description"
+   ```
+
+3. **Push to your fork**:
+   ```bash
+   git push origin feature/your-feature-name
+   ```
+
+4. **Open a Pull Request**:
+   - Go to the original repository
+   - Click "New Pull Request"
+   - Select your branch
+   - Fill in the PR description template
+   - Reference any related issues
+
+## PR Guidelines
+
+- Keep PRs focused on a single change
+- Include tests for new features
+- Update documentation if needed
+- Ensure all tests pass
+- Be responsive to review feedback
+
+## Questions?
+
+Feel free to open an issue for any questions or discussions about contributing.