feat: add image generation support with OpenAI image generation model

## CHANGES

- Add `--image-file` flag for saving generated images
- Implement image generation tool integration with OpenAI
- Support image editing with attachment input files
- Add comprehensive test coverage for image features
- Update documentation with image generation examples
- Fix HTML formatting issues in README
- Improve PowerShell code block indentation
- Clean up help text formatting and spacing

common/domain.go

@@ -28,6 +28,7 @@ type ChatOptions struct {
 	MaxTokens          int
 	Search             bool
 	SearchLocation     string
+	ImageFile          string
 }
 
 // NormalizeMessages remove empty messages and ensure messages order user-assist-user