feat: implement design tools

## Summary

Design tools (image generation, mockup tools) are listed in the tool categories but not implemented.

## Design Spec Reference

- **§11.1** Tool Categories — Design row

## Scope

- Image generation tool (technology TBD)
- Mockup/wireframe tool
- `BaseTool` compliance with `ToolCategory.DESIGN`

## Notes

- Specific tooling and APIs to be evaluated at implementation time
