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/crud.md

@@ -0,0 +1,14 @@
+# Pattern: CRUD slice
+
+For a resource `Foo`:
+
+1. Migration `foo` table (`id, created_at, updated_at, deleted_at`, …)
+2. `model.Foo`
+3. `repository` methods: Get / List / Create / Update / SoftDelete
+4. `service` with authz + validation
+5. `handler` REST routes
+6. OpenAPI paths
+7. SDK methods + types
+8. UI only if user-facing
+
+List endpoints always paginate (`page`, `page_size`, `total`).