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/push.go

@@ -0,0 +1,69 @@
+// 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/print"
+	pullstack "gitea.dev/tea/modules/stack"
+	"gitea.dev/tea/modules/task"
+	"gitea.dev/tea/modules/utils"
+
+	"github.com/urfave/cli/v3"
+)
+
+// CmdStackPush represents a sub command to link a pull request onto a stack
+var CmdStackPush = cli.Command{
+	Name:        "push",
+	Usage:       "Link a pull request onto a stack",
+	Description: "Link a child pull request onto its parent, creating a new stack or extending an existing one with --stack",
+	ArgsUsage:   "<parent index> <child index>",
+	Action:      RunStackPush,
+	Flags: append([]cli.Flag{
+		&cli.Int64Flag{
+			Name:  "stack",
+			Usage: "id of an existing stack to append the pull requests to, instead of creating a new stack",
+		},
+	}, flags.AllDefaultFlags...),
+}
+
+// RunStackPush links a parent and a child pull request into a stack
+func RunStackPush(requestCtx stdctx.Context, cmd *cli.Command) error {
+	if cmd.Args().Len() != 2 {
+		return fmt.Errorf("parent index and child index are required")
+	}
+	parentIndex, err := utils.ArgToIndex(cmd.Args().Get(0))
+	if err != nil {
+		return err
+	}
+	childIndex, err := utils.ArgToIndex(cmd.Args().Get(1))
+	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
+	}
+
+	var s *pullstack.Stack
+	if stackID := cmd.Int64("stack"); stackID != 0 {
+		s, err = task.AddPullStackEntry(requestCtx, ctx.Login, ctx.Owner, ctx.Repo, stackID, parentIndex, childIndex)
+	} else {
+		s, err = task.CreatePullStack(requestCtx, ctx.Login, ctx.Owner, ctx.Repo, parentIndex, childIndex)
+	}
+	if err != nil {
+		return err
+	}
+
+	print.PullStackDetail(s)
+	return nil
+}