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/service.md

@@ -0,0 +1,31 @@
+# Pattern: Service
+
+## Responsibility
+
+Business rules, authorization, orchestration, transactions.
+
+## Shape
+
+```go
+type ReportRepo interface {
+    SaveReport(ctx context.Context, r *model.Report) error
+}
+
+type ReportService struct {
+    repo ReportRepo
+}
+
+func (s *ReportService) Decode(ctx context.Context, in DecodeInput) (*DecodeOutput, error) {
+    if err := in.Validate(); err != nil {
+        return nil, fmt.Errorf("validate decode: %w", err)
+    }
+    // rules / engine
+    // persist via repo
+    return out, nil
+}
+```
+
+## Never
+
+- Import `gin`
+- Bypass repo with ad-hoc SQL drivers scattered around