Proposal: include StitchFlow in copilot-plugins

I'd like to propose [StitchFlow](https://github.com/yshishenya/stitchflow) for the official Copilot plugins collection.

StitchFlow already ships a GitHub Copilot plugin and packages Google Stitch into a reusable workflow that turns product briefs into UI directions, local HTML, and screenshots.

Why it may fit:
- clear Copilot plugin packaging already in repo
- practical design/dev workflow, not just a prompt pack
- cross-agent support also helps discovery beyond Copilot

Repo includes live demo assets and a polished README.