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/repository.md

@@ -0,0 +1,24 @@
+# Pattern: Repository
+
+## Responsibility
+
+Persistence only. Parameterized SQL. Map rows ↔ model.
+
+## Shape
+
+```go
+func (r *ReportRepository) SaveReport(ctx context.Context, m *model.Report) error {
+    const q = `INSERT INTO report (id, user_id, kind, payload, created_at, updated_at)
+               VALUES ($1,$2,$3,$4,now(),now())`
+    _, err := r.db.ExecContext(ctx, q, m.ID, m.UserID, m.Kind, m.Payload)
+    if err != nil {
+        return fmt.Errorf("insert report: %w", err)
+    }
+    return nil
+}
+```
+
+## Never
+
+- `SELECT *` in new code
+- Business policy (VIP checks) here — return data, let service decide