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/examples/good-service.go

@@ -0,0 +1,29 @@
+//go:build ignore
+
+// EXAMPLE — reference shape for AI. Not compiled into apps/api.
+package examples
+
+import (
+	"context"
+	"fmt"
+)
+
+type DecodeRepo interface {
+	SaveDecode(ctx context.Context, userID string, summary string) error
+}
+
+type DecodeServiceImpl struct {
+	Repo DecodeRepo
+}
+
+func (s *DecodeServiceImpl) Decode(ctx context.Context, in DecodeInput) (*DecodeOutput, error) {
+	if in.Year < 1900 || in.Month < 1 || in.Month > 12 || in.Day < 1 || in.Day > 31 {
+		return nil, fmt.Errorf("invalid birth date")
+	}
+	// engine would run here
+	summary := "简版结论占位"
+	if err := s.Repo.SaveDecode(ctx, "user-from-ctx", summary); err != nil {
+		return nil, fmt.Errorf("save decode: %w", err)
+	}
+	return &DecodeOutput{Summary: summary}, nil
+}