feat(pulls): support branch-validated stack reordering

Assisted-by: Codet:GPT-5-Codex

cmd/pulls/stack/reorder_test.go

@@ -0,0 +1,17 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package stack
+
+import (
+	"context"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestStackReorderRequiresCompleteOrder(t *testing.T) {
+	cmd := CmdStackReorder
+	err := cmd.Run(context.Background(), []string{"reorder", "5", "2"})
+	assert.ErrorContains(t, err, "at least two pull numbers")
+}