feat(pulls): add tea pr stack list|show|push|restack|merge

Adds a CLI client for the Gitea EE pull request stack API
(GET/POST /repos/{owner}/{repo}/stacks[/{id}[/entries|/restack|/merge]]):

- modules/stack holds the Stack/Entry/MergeResult types shared between
  the API client and its output formatting, avoiding an import cycle
  between modules/task and modules/print.
- modules/task/pull_stack.go implements the HTTP calls on top of
  modules/api.Client, which gains a context-aware DoContext method.
- modules/print/pull_stack.go formats stack listings (table/json/...)
  and a single stack's ordered entries.
- cmd/pulls/stack adds the list, show, push, restack and merge
  subcommands, wired in as 'tea pr stack' via cmd/pulls.go, following
  the repo/login flag routing used by the rest of the pulls commands.

Assisted-by: Codet:GPT-5-Codex

cmd/pulls/stack/merge_test.go

@@ -0,0 +1,32 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package stack
+
+import (
+	"context"
+	"testing"
+
+	"gitea.dev/tea/modules/config"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestStackMergeRequiresStackID(t *testing.T) {
+	config.SetConfigForTesting(config.LocalConfig{
+		Logins: []config.Login{{
+			Name:    "testLogin",
+			URL:     "https://gitea.example.com",
+			Token:   "test-token",
+			User:    "testUser",
+			Default: true,
+		}},
+	})
+	t.Cleanup(func() {
+		config.SetConfigForTesting(config.LocalConfig{})
+	})
+
+	cmd := CmdStackMerge
+	err := cmd.Run(context.Background(), []string{"merge", "--login", "testLogin", "--repo", "user/repo"})
+	assert.ErrorContains(t, err, "stack id is required")
+}