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>

.ai/checklists/refactor.md

@@ -0,0 +1,8 @@
+# Checklist: Refactor
+
+- [ ] Goal / non-goals stated (behavior unchanged unless asked)
+- [ ] No ADR silently reversed
+- [ ] Layers still clean
+- [ ] File/function size improved toward limits
+- [ ] Tests still pass; added if extracting unscored logic
+- [ ] Review: Architecture / Readability / Test / Scope