fix(edit): point empty-replace error at operation=delete

A replace with empty new_content failed with a bare 'new_content is required' message, leaving the model to guess. Append a hint to use operation=delete when the intent is removal. Adds TestValidateReplaceEmptySuggestsDelete.

internal/edit/edit.go

@@ -270,7 +270,11 @@ func (e *Engine) validateBaseEdit(edit *ASTEdit) error {
 	switch edit.Operation {
 	case EditReplace, EditInsertBefore, EditInsertAfter:
 		if edit.NewContent == "" {
-			return errors.NewInvalidEditError(fmt.Sprintf("new_content is required for %s operation", edit.Operation))
+			msg := fmt.Sprintf("new_content is required for %s operation", edit.Operation)
+			if edit.Operation == EditReplace {
+				msg += " (to remove content instead, use operation=delete)"
+			}
+			return errors.NewInvalidEditError(msg)
 		}
 	case EditDelete:
 		// new_content not required

internal/edit/edit_test.go

@@ -101,6 +101,22 @@ func TestValidateEdit(t *testing.T) {
 	}
 }
 
+// A replace with empty new_content should point the caller at operation=delete.
+func TestValidateReplaceEmptySuggestsDelete(t *testing.T) {
+	e := NewEngine(parser.NewRegistry())
+	err := e.validateASTEdit(&ASTEdit{
+		File:      "test.go",
+		Operation: EditReplace,
+		Selector:  ASTSelector{Kind: "function_declaration"},
+	})
+	if err == nil {
+		t.Fatal("expected error for replace with empty new_content")
+	}
+	if !strings.Contains(err.Error(), "operation=delete") {
+		t.Errorf("error should suggest operation=delete, got: %v", err)
+	}
+}
+
 func TestResolveSelector(t *testing.T) {
 	registry := parser.NewRegistry()
 	defer registry.Close()