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.

檢查 StitchFlow 儲存庫、其 Copilot 外掛程式的封裝方式、即時示範資產和 README，然後將它們與 copilot-plugins 儲存庫的納入要求進行比較。完成的標準是確定 StitchFlow 是否應加入官方集合，並記錄最終決定或後續步驟。

由索引模型根據 Issue 內容生成。