Create guide and evals for the webmcp use case

[chrome-devrel-infra]

Use this guide to implement WebMCP (Web Model Context Protocol) in browser applications, exposing client-side functionality as tools to AI agents via declarative or imperative APIs.

Affected web-feature IDs:

Use case subdir: guides/webmcp/webmcp

Required files:

• Use case metadata (guide.md frontmatter)
• demo.html
• Use case guidance (guide.md)
• expectations.md
• tasks/task.md
• negative-demo.html
• grader.ts