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.go

@@ -0,0 +1,71 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package print
+
+import (
+	"fmt"
+	"strings"
+
+	"gitea.dev/tea/modules/stack"
+)
+
+// PullStackFields are all available fields to print with PullStacksList()
+var PullStackFields = []string{
+	"id",
+	"title",
+	"base",
+	"state",
+	"auto-restack",
+	"entries",
+}
+
+// PullStacksList prints a listing of pull request stacks
+func PullStacksList(stacks []*stack.Stack, output string, fields []string) error {
+	printables := make([]printable, len(stacks))
+	for i, s := range stacks {
+		printables[i] = &printablePullStack{s}
+	}
+	t := tableFromItems(fields, printables, isMachineReadable(output))
+	return t.print(output)
+}
+
+type printablePullStack struct {
+	*stack.Stack
+}
+
+func (x printablePullStack) FormatField(field string, machineReadable bool) string {
+	switch field {
+	case "id":
+		return fmt.Sprintf("%d", x.ID)
+	case "title":
+		return x.Title
+	case "base":
+		return x.BaseBranch
+	case "state":
+		return x.State
+	case "auto-restack":
+		return formatBoolean(x.AutoRestack, !machineReadable)
+	case "entries":
+		return fmt.Sprintf("%d", len(x.Entries))
+	}
+	return ""
+}
+
+// PullStackDetail prints a pull request stack, bottom entry first, rendered to stdout
+func PullStackDetail(s *stack.Stack) {
+	out := fmt.Sprintf(
+		"# Stack #%d: %s (%s)\nbase: **%s**\tauto-restack: %s\n\n",
+		s.ID, s.Title, s.State, s.BaseBranch, formatBoolean(s.AutoRestack, true),
+	)
+
+	for _, entry := range s.Entries {
+		if entry.ParentPullID == 0 {
+			out += fmt.Sprintf("- #%d (%s)\n", entry.PullID, entry.State)
+		} else {
+			out += fmt.Sprintf("- #%d <- #%d (%s)\n", entry.PullID, entry.ParentPullID, entry.State)
+		}
+	}
+
+	outputMarkdown(strings.TrimRight(out, "\n")+"\n", "")
+}