feat(mcp): live page preview tool — render and screenshot agent-authored content

[perasperaactual]

Problem

The MCP server can write valid YAML (write_page) and show pre-canned component screenshots (preview_component), but an agent cannot see the rendered result of the content it actually wrote. The agent composes blind — it knows the schema is valid but can't verify visual correctness.

This is the gap between "schema-valid" and "looks right."

Why it matters

This is the single highest-leverage feature for a hackathon demo. It turns the story from "trust me, the schema validated" into "look at what the AI just built." It completes the AI authoring loop: introspect schema → write content → see rendered result → iterate.

Depends on

• fix(build-scripts): hot recompilation on YAML and image content changes #122 (hot recompilation) — the dev server must pick up YAML changes for preview to work

Proposed approach

Add a stackwright_preview_page MCP tool that:

1. Accepts a slug parameter (page must already exist via write_page or add_page)
2. Ensures the dev server is running (or starts one)
3. Triggers a prebuild for the target page (or relies on the watcher from fix(build-scripts): hot recompilation on YAML and image content changes #122)
4. Uses Playwright to navigate to the page URL and take a full-page screenshot
5. Returns the screenshot as an MCP image content block

The infrastructure is 90% there:

• Playwright is already a dev dependency (E2E + visual regression tests)
• The screenshot pipeline exists in packages/e2e/
• The prebuild system exists in @stackwright/build-scripts

Open questions

• Should the tool manage its own dev server lifecycle, or require the user to have pnpm dev running?
• Should it support inline YAML (write to a temp page, screenshot, clean up) for "what would this look like?" queries?
• Desktop-only screenshot, or also mobile viewport?

Acceptance criteria

• Agent calls write_page then preview_page and receives a screenshot of the rendered page
• Screenshot reflects the actual YAML content, not stale/cached content
• Works with co-located images
• Returns useful error if dev server is not running or page doesn't exist
• Response time under 10s for a single page preview