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

modules/print/pull_stack_test.go

@@ -0,0 +1,61 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package print
+
+import (
+	"bytes"
+	"encoding/json"
+	"testing"
+
+	"gitea.dev/tea/modules/stack"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestPullStacksListJSON(t *testing.T) {
+	stacks := []*stack.Stack{
+		{
+			ID: 1, Title: "feat: base", BaseBranch: "main", State: "open", AutoRestack: true,
+			Entries: []stack.Entry{{PullID: 10}, {PullID: 11}},
+		},
+	}
+
+	buf := &bytes.Buffer{}
+	tbl := tableFromItems(
+		PullStackFields,
+		[]printable{&printablePullStack{stacks[0]}},
+		true,
+	)
+	require.NoError(t, tbl.fprint(buf, "json"))
+
+	var result []map[string]string
+	require.NoError(t, json.Unmarshal(buf.Bytes(), &result))
+	require.Len(t, result, 1)
+	assert.Equal(t, "1", result[0]["id"])
+	assert.Equal(t, "feat: base", result[0]["title"])
+	assert.Equal(t, "main", result[0]["base"])
+	assert.Equal(t, "open", result[0]["state"])
+	assert.Equal(t, "true", result[0]["auto-restack"])
+	assert.Equal(t, "2", result[0]["entries"])
+}
+
+func TestPullStackDetailShowsEntries(t *testing.T) {
+	s := &stack.Stack{
+		ID: 3, Title: "feat: base", BaseBranch: "main", State: "open", AutoRestack: true,
+		Entries: []stack.Entry{
+			{PullID: 10, Position: 0, State: "merged"},
+			{PullID: 11, ParentPullID: 10, Position: 1, State: "open"},
+		},
+	}
+
+	out := captureStdout(t, func() {
+		PullStackDetail(s)
+	})
+
+	assert.Contains(t, out, "feat: base")
+	assert.Contains(t, out, "main")
+	assert.Contains(t, out, "#10")
+	assert.Contains(t, out, "#11")
+}