feat(mini-apps): add code analysis, research, and vibe coding demos

[Jonnytoshen]

Summary

• add the fetch_url and start_preview tools and register them in the tool system
• add Mini Apps demo flows for code analysis, research, and vibe coding across the CLI, mock model, sample project, and app bootstrap
• sync the README with the new demos, tool surface, and preview workflow

Testing

• pnpm preflight