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/adr/0004-use-jwt.md

@@ -0,0 +1,19 @@
+# ADR-0004 JWT (or opaque Bearer) for auth
+
+## Status
+
+Accepted (direction)
+
+## Decision
+
+Clients send `Authorization: Bearer <token>`. Prefer JWT for stateless MVP; server may later move to opaque tokens + session store via new ADR.
+
+## Reason
+
+1. Works for H5 and future mini-program
+2. Simple middleware story
+
+## Never
+
+Never invent parallel auth headers (`X-User-Id` as sole auth).  
+Never trust client-only “isVip” flags.