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/0003-rest-api.md

@@ -0,0 +1,20 @@
+# ADR-0003 REST API (not GraphQL)
+
+## Status
+
+Accepted
+
+## Decision
+
+Public HTTP API is REST under `/api/v1` with envelope `{code,message,data}`.
+
+## Reason
+
+1. Simpler mental model for H5 + mini-program
+2. Better Cursor/Claude reliability than GraphQL schemas mid-flight
+3. Easier debugging with curl
+
+## Never
+
+Never replace with GraphQL without approval + new ADR.  
+Internal gRPC later is allowed only behind a new ADR; public clients stay REST.