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>

deploy/.env.example

@@ -0,0 +1,4 @@
+APP_ENV=dev
+HTTP_ADDR=:8080
+DATABASE_URL=postgres://yuxingu:yuxingu@127.0.0.1:5432/yuxingu?sslmode=disable
+JWT_SECRET=change-me-in-production

deploy/docker-compose.yml

@@ -0,0 +1,15 @@
+services:
+  postgres:
+    image: postgres:16-alpine
+    container_name: yuxingu-postgres
+    environment:
+      POSTGRES_USER: yuxingu
+      POSTGRES_PASSWORD: yuxingu
+      POSTGRES_DB: yuxingu
+    ports:
+      - "5432:5432"
+    volumes:
+      - yuxingu_pg:/var/lib/postgresql/data
+
+volumes:
+  yuxingu_pg: