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/stack/stack.go

@@ -0,0 +1,33 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+// Package stack holds the data types shared between the pull request stack API
+// client (modules/task) and its output formatting (modules/print).
+package stack
+
+// Entry is one pull request participating in a pull request stack.
+type Entry struct {
+	PullID           int64  `json:"pull_id"`
+	ParentPullID     int64  `json:"parent_pull_id"`
+	Position         int    `json:"position"`
+	State            string `json:"state"`
+	LastRestackedSHA string `json:"last_restacked_sha"`
+}
+
+// Stack is a same-repository chain of pull requests, bottom first.
+type Stack struct {
+	ID          int64   `json:"id"`
+	RepoID      int64   `json:"repo_id"`
+	OwnerID     int64   `json:"owner_id"`
+	Title       string  `json:"title"`
+	BaseBranch  string  `json:"base_branch"`
+	State       string  `json:"state"`
+	AutoRestack bool    `json:"auto_restack"`
+	Entries     []Entry `json:"entries"`
+}
+
+// MergeResult reports how many pull requests of a stack were merged.
+type MergeResult struct {
+	Merged int64  `json:"merged"`
+	Error  string `json:"error,omitempty"`
+}