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.

Review the StitchFlow repository, its Copilot plugin packaging, live demo assets, and README, then compare them with the copilot-plugins repository's inclusion expectations. Done means determining whether StitchFlow should be added to the official collection and documenting the resulting decision or next steps.

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