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.go

@@ -0,0 +1,73 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package stack
+
+import (
+	stdctx "context"
+	"fmt"
+
+	"gitea.dev/tea/cmd/flags"
+	"gitea.dev/tea/modules/context"
+	"gitea.dev/tea/modules/task"
+	"gitea.dev/tea/modules/utils"
+
+	"github.com/urfave/cli/v3"
+)
+
+// CmdStackMerge represents a sub command to merge a pull request stack in order
+var CmdStackMerge = cli.Command{
+	Name:        "merge",
+	Aliases:     []string{"m"},
+	Usage:       "Merge a pull request stack in order",
+	Description: "Merge every pull request of a stack in order, bottom first",
+	ArgsUsage:   "<stack id>",
+	Action:      RunStackMerge,
+	Flags: append([]cli.Flag{
+		&cli.StringFlag{
+			Name:    "style",
+			Aliases: []string{"s"},
+			Usage:   "Kind of merge to perform: merge, rebase, squash, rebase-merge",
+			Value:   "merge",
+		},
+		&cli.BoolFlag{
+			Name:  "wait-for-ci",
+			Usage: "Schedule the merge to run once required checks pass, instead of merging immediately",
+		},
+	}, flags.AllDefaultFlags...),
+}
+
+// RunStackMerge merges every pull request of a stack in order
+func RunStackMerge(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
+	}
+
+	result, queued, err := task.MergePullStack(requestCtx, ctx.Login, ctx.Owner, ctx.Repo, id,
+		cmd.String("style"), cmd.Bool("wait-for-ci"))
+	if queued {
+		fmt.Fprintf(ctx.Writer, "merge for stack #%d scheduled\n", id)
+		return nil
+	}
+	if err != nil {
+		if result != nil {
+			return fmt.Errorf("merged %d pull request(s) before failing: %w", result.Merged, err)
+		}
+		return err
+	}
+
+	fmt.Fprintf(ctx.Writer, "merged %d pull request(s) of stack #%d\n", result.Merged, id)
+	return nil
+}