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/bugfix.md

@@ -0,0 +1,9 @@
+# Checklist: Bugfix
+
+- [ ] Failure mode stated
+- [ ] Root cause located (file/func)
+- [ ] Minimal fix only
+- [ ] Regression test if scoring/auth/payment
+- [ ] No unrelated refactors
+- [ ] Build/tests green
+- [ ] Review: Architecture / Security / Test / Scope