chore(go): add failing core constructor tests

internal/homesick/core/core_test.go

@@ -1,6 +1,31 @@
 package core
 
-import "testing"
+import (
+	"bytes"
+	"testing"
+)
+
+func TestNewRejectsNilWriters(t *testing.T) {
+	t.Run("nil stdout", func(t *testing.T) {
+		app, err := New(nil, &bytes.Buffer{})
+		if err == nil {
+			t.Fatal("expected error for nil stdout")
+		}
+		if app != nil {
+			t.Fatal("expected nil app for nil stdout")
+		}
+	})
+
+	t.Run("nil stderr", func(t *testing.T) {
+		app, err := New(&bytes.Buffer{}, nil)
+		if err == nil {
+			t.Fatal("expected error for nil stderr")
+		}
+		if app != nil {
+			t.Fatal("expected nil app for nil stderr")
+		}
+	})
+}
 
 func TestDeriveDestination(t *testing.T) {
 	tests := []struct {