feat: support DESIGN.md workflow for design system creation

[davideast]

We recently added the ability to create design systems from markdown files (DESIGN.md). This is a two-step workflow:

1. Upload a markdown file describing the design language (colors, typography, spacing, etc.) to a project
2. Create a design system from that uploaded file, which Stitch interprets and converts into actionable design tokens

SDK (domain classes)

const instance = await project.uploadDesignMd(btoa(myDesignMarkdown));
const ds = await project.createDesignSystemFromDesignMd(instance, "DESKTOP");
await ds.apply(screenInstances);

Agent (callTool / tool definitions)

Both tools are also registered in the tool definitions, so agents using callTool or the ADK/AI SDK adapters can discover and invoke them directly:

const instance = await client.callTool("upload_design_md", {
  projectId: "123",
  designMdBase64: btoa(markdown),
});

const ds = await client.callTool("create_design_system_from_design_md", {
  projectId: "123",
  selectedScreenInstance: { id: instance.id, sourceScreen: instance.sourceScreen },
  deviceType: "DESKTOP",
});

This brings the SDK to full MCP tool coverage (14/14) across both modalities.