feat(handler): implement default sort configuration for models

* Add SetDefaultSort method to configure default sort order
* Implement getDefaultSort method to retrieve configured defaults
* Update handleRead to apply default sort when none specified
* Add tests for default sort functionality

pkg/restheadspec/default_sort_test.go

@@ -0,0 +1,38 @@
+package restheadspec
+
+import (
+	"reflect"
+	"testing"
+
+	"github.com/bitechdev/ResolveSpec/pkg/common"
+)
+
+func TestSetDefaultSort(t *testing.T) {
+	handler := NewHandler(nil, nil)
+
+	// No default configured yet
+	if got := handler.getDefaultSort("public", "users"); got != nil {
+		t.Errorf("Expected no default sort, got %v", got)
+	}
+
+	// Global default
+	global := []common.SortOption{{Column: "created_at", Direction: "desc"}}
+	handler.SetDefaultSort("", "", global...)
+
+	if got := handler.getDefaultSort("public", "users"); !reflect.DeepEqual(got, global) {
+		t.Errorf("Expected global default sort %v, got %v", global, got)
+	}
+
+	// Per-model default overrides the global default
+	perModel := []common.SortOption{{Column: "name", Direction: "asc"}}
+	handler.SetDefaultSort("public", "users", perModel...)
+
+	if got := handler.getDefaultSort("public", "users"); !reflect.DeepEqual(got, perModel) {
+		t.Errorf("Expected per-model default sort %v, got %v", perModel, got)
+	}
+
+	// Other models still fall back to the global default
+	if got := handler.getDefaultSort("public", "orders"); !reflect.DeepEqual(got, global) {
+		t.Errorf("Expected global default sort %v for unrelated model, got %v", global, got)
+	}
+}