feat: Image input support for edit_screens / new review_screen tool

[grahama1970]

Problem

The edit_screens and generate_screen_from_text tools only accept text prompts. There's no way to send an image (e.g., a screenshot of a built component) to Stitch for visual comparison against a design target.

Stitch's web UI supports this through "Experimental Mode" — upload a screenshot, get AI-powered review with annotated feedback. But the MCP server doesn't expose image input.

Use Case: Design-vs-Implementation Review Loop

After an agent builds a React component from a Stitch mockup, it needs Stitch to verify the implementation matches the design:

1. /mockup-lab generate → approved Stitch mockup (design target)
2. Agent builds React component
3. Agent screenshots the running component
4. Agent sends screenshot to Stitch: "Compare this against the design target"
5. Stitch returns annotated correction mockup showing diffs
6. Agent fixes code → repeat until Stitch approves

Step 4 is impossible via the SDK because edit_screens only accepts text, not images.

Proposed Solutions

Option A: Add imageData (base64) or imageUrl parameter to edit_screens:

{
  "projectId": "...",
  "selectedScreenIds": ["design-target-id"],
  "prompt": "Compare this implementation against the design target",
  "referenceImage": "<base64 PNG or URL>"
}

Option B: New dedicated review_screen tool:

{
  "projectId": "...",
  "screenId": "design-target-id",
  "implementationImage": "<base64 PNG or URL>",
  "prompt": "List every difference: spacing, colors, typography, layout"
}

Returns an annotated correction mockup + structured diff (pixel values, hex colors, missing elements).

Context

This is a companion to #129 (collaboration API). Together they would enable:

• Human feedback via comments API (feat: Collaboration API tools (comments, chat history) for agentic workflows #129)
• Visual implementation review via image input (this issue)
• Full agentic design → build → review → iterate loop without leaving the SDK

Environment

• @google/stitch-sdk 0.0.3
• Discovered while building an agentic design pipeline that generates mockups, codes React components, and needs Stitch to verify the implementation.