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/playbooks/new-table.md

@@ -0,0 +1,14 @@
+# Playbook: New Table
+
+Use when: “新建表 / 加字段”
+
+## Steps
+
+1. Name with `snake_case` (`.ai/database.md` + `.ai/domain.md`)
+2. Columns include `id, created_at, updated_at, deleted_at`
+3. Write migration Up (+ Down)
+4. Index FKs
+5. Update `model` + repository
+6. Never ship app code that queries columns not migrated
+7. Document in PR
+8. Review (Deployment + Docs items)