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/feature.md

@@ -0,0 +1,16 @@
+# Checklist: Feature
+
+- [ ] Architecture layers respected
+- [ ] Domain words match `.ai/domain.md`
+- [ ] API (+ OpenAPI if public)
+- [ ] Service
+- [ ] Repository (if persistence)
+- [ ] Migration (if schema)
+- [ ] SDK/types (if client needs)
+- [ ] UI (if user-facing)
+- [ ] Test or smoke steps
+- [ ] Docs touched if behavior user-visible
+- [ ] `go test` / `npm run build:h5` as applicable
+- [ ] Health still OK
+- [ ] Review block printed
+- [ ] DoD satisfied