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-test.go

@@ -0,0 +1,39 @@
+//go:build ignore
+
+// EXAMPLE — table-driven unit test shape.
+package examples
+
+import "testing"
+
+func TestClampBirthMonth(t *testing.T) {
+	tests := []struct {
+		name    string
+		month   int
+		wantErr bool
+	}{
+		{name: "ok", month: 6, wantErr: false},
+		{name: "zero", month: 0, wantErr: true},
+		{name: "thirteen", month: 13, wantErr: true},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := validateMonth(tt.month)
+			if (err != nil) != tt.wantErr {
+				t.Fatalf("validateMonth(%d) err=%v wantErr=%v", tt.month, err, tt.wantErr)
+			}
+		})
+	}
+}
+
+func validateMonth(m int) error {
+	if m < 1 || m > 12 {
+		return errInvalid
+	}
+	return nil
+}
+
+var errInvalid = errString("invalid")
+
+type errString string
+
+func (e errString) Error() string { return string(e) }