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/show.go

@@ -0,0 +1,62 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package stack
+
+import (
+	stdctx "context"
+	"encoding/json"
+	"fmt"
+
+	"gitea.dev/tea/cmd/flags"
+	"gitea.dev/tea/modules/context"
+	"gitea.dev/tea/modules/print"
+	"gitea.dev/tea/modules/task"
+	"gitea.dev/tea/modules/utils"
+
+	"github.com/urfave/cli/v3"
+)
+
+// CmdStackShow represents a sub command to show a pull request stack
+var CmdStackShow = cli.Command{
+	Name:        "show",
+	Aliases:     []string{"get"},
+	Usage:       "Show a pull request stack",
+	Description: "Show a pull request stack, its base branch and its ordered pull requests",
+	ArgsUsage:   "<stack id>",
+	Action:      RunStackShow,
+	Flags:       flags.AllDefaultFlags,
+}
+
+// RunStackShow shows a pull request stack
+func RunStackShow(requestCtx stdctx.Context, cmd *cli.Command) error {
+	if cmd.Args().Len() != 1 {
+		return fmt.Errorf("stack id is required")
+	}
+	id, err := utils.ArgToIndex(cmd.Args().First())
+	if err != nil {
+		return err
+	}
+
+	ctx, err := context.InitCommand(cmd)
+	if err != nil {
+		return err
+	}
+	if err := ctx.Ensure(context.CtxRequirement{RemoteRepo: true}); err != nil {
+		return err
+	}
+
+	s, err := task.GetPullStack(requestCtx, ctx.Login, ctx.Owner, ctx.Repo, id)
+	if err != nil {
+		return err
+	}
+
+	if ctx.String("output") == "json" {
+		encoder := json.NewEncoder(ctx.Writer)
+		encoder.SetIndent("", "\t")
+		return encoder.Encode(s)
+	}
+
+	print.PullStackDetail(s)
+	return nil
+}