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/patterns/jwt.md

@@ -0,0 +1,13 @@
+# Pattern: JWT / Bearer
+
+## Middleware
+
+1. Read `Authorization: Bearer <token>`
+2. Parse/verify
+3. Put `user_id` into context
+4. Reject with code in 1xxxx if missing/invalid
+
+## Service
+
+Always take `userID` from context for mutating ops.  
+Never trust body `user_id` from client for ownership.