Add copilot configuration and developer guides

- Created copilot_config.md with configuration examples
- Created copilot_developer_guide.md for extending copilot
- Documented architecture, testing, and best practices
- Added examples for adding new modes and features

Co-authored-by: HeDaas-Code <208586641+HeDaas-Code@users.noreply.github.com>

Author: Copilot

docs/copilot_config.md

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+# MGit Copilot Configuration Example
+
+This file shows example configuration for the Copilot feature.
+
+## Configuration Location
+
+The copilot configuration is stored in MGit's config file (managed by ConfigManager).
+Typically located at: `~/.config/mgit/config.json` or similar location depending on your OS.
+
+## Configuration Keys
+
+```json
+{
+  "copilot": {
+    "enabled": true,
+    "api_key": "your-siliconflow-api-key-here",
+    "model": "Qwen/Qwen2.5-7B-Instruct"
+  }
+}
+```
+
+## Available Models
+
+You can use any of these models with SiliconFlow:
+
+### Qwen Series (Recommended)
+- `Qwen/Qwen2.5-7B-Instruct` - Best balance of speed and quality (default)
+- `Qwen/Qwen2.5-14B-Instruct` - Higher quality, slightly slower
+- `Qwen/Qwen2.5-32B-Instruct` - Highest quality, slower
+
+### DeepSeek
+- `deepseek-ai/DeepSeek-V2.5` - Strong reasoning, good for technical content
+
+### Meta LLaMA
+- `meta-llama/Meta-Llama-3.1-8B-Instruct` - Open source alternative
+
+## Getting API Key
+
+1. Visit https://siliconflow.cn
+2. Register or login to your account
+3. Navigate to API Keys section
+4. Create a new API key
+5. Copy the key to Copilot settings in MGit
+
+## Configuration via UI
+
+The recommended way to configure Copilot is through the UI:
+
+1. Open MGit
+2. Go to menu: `Copilot > Copilot设置`
+3. Enter your API key
+4. Select your preferred model
+5. Check "启用 Copilot"
+6. Click "测试连接" to verify
+7. Click "保存"
+
+## Security Notes
+
+- API keys are stored in local configuration file
+- Never commit configuration files with API keys to git
+- Use environment variables for CI/CD if needed
+- Keys are encrypted in configuration storage
+
+## Environment Variables (Optional)
+
+You can also set API key via environment variable:
+
+```bash
+export SILICONFLOW_API_KEY="your-api-key-here"
+```
+
+Note: UI configuration takes precedence over environment variables.
+
+## Troubleshooting
+
+### API Key Not Working
+- Verify key is correct (no extra spaces)
+- Check account balance on SiliconFlow
+- Ensure network connectivity
+- Try "测试连接" in settings dialog
+
+### Model Not Available
+- Some models may require specific API tier
+- Check SiliconFlow documentation for model availability
+- Try using default model first
+
+### Connection Timeout
+- Check internet connection
+- Verify firewall settings
+- Try increasing timeout in code if needed
+
+## Advanced Configuration
+
+For advanced users, you can modify additional parameters by editing the source code:
+
+- `temperature`: Controls randomness (0.0-1.0), in copilot_manager.py
+- `max_tokens`: Maximum output length, in copilot_manager.py
+- API timeout: In siliconflow_client.py
+
+## Rate Limits
+
+Be aware of SiliconFlow API rate limits:
+- Free tier: Limited requests per minute
+- Paid tiers: Higher limits
+
+Check your plan at https://siliconflow.cn
+
+## Support
+
+For issues or questions:
+- Check the [Copilot Guide](./copilot_guide.md)
+- Submit GitHub issue
+- Contact SiliconFlow support for API issues