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/release.md

@@ -0,0 +1,10 @@
+# Checklist: Release
+
+- [ ] main green: `go test ./...`, `npm run build:h5`
+- [ ] Migrations applied plan documented
+- [ ] Health endpoint verified
+- [ ] No secrets in artifact
+- [ ] Image tag ≠ `latest` (prod)
+- [ ] OpenAPI matches deployed routes
+- [ ] Changelog / version tag if shipping
+- [ ] Rollback path known