chore: seal Design Vision v1 and monorepo scaffold

Archive the differentiated YuXinGu product docs, AI engineering system,
design contract, and Go/Vue scaffold. Next execution prioritizes Cece-parity
over early innovation (see .ai/product/STRATEGY.md).

Co-authored-by: Cursor <cursoragent@cursor.com>

LEGACY.md

@@ -0,0 +1,9 @@
+# Legacy 原型说明
+
+以下路径为重构前的静态 H5 原型，**默认只读**，新功能请在 `apps/user-h5` 与 `apps/api` 开发：
+
+- `yuxingu.html`、`index.html`
+- `pages/`、`css/`、`js/`
+- `server.py`（Python 静态站 + 量表 API）
+
+可对照交互与文案，迁移完成后移入 `archive/legacy-h5/`。