Merge Stacked PRs with merge command (#307)

* merge cmd

* Refine the merge TUI and simplify the async-merge client

Follow-up polish for `gh stack merge` (the command itself landed in the
previous commit). These changes refine the interactive wizard, enrich the
PR picker, and replace the merge client's bespoke HTTP handling with the
standard go-gh REST client.

Wizard and stepper:
- Redesign the top stepper as a segmented bar: completed steps are green,
  the active step is the brightest, and upcoming steps are dimmed. Steps
  are separated by a Powerline arrow that blends into the shading, with a
  graceful fallback to abutting segments on terminals that lack the glyph
  (e.g. Apple Terminal). Set GH_STACK_POWERLINE=1/0 to override detection.
- Show the stack number in the header ("Merge stack #123").
- Hide the header and stepper once the merge is submitted so the live
  progress view stands on its own.

PR picker:
- Render each pull request on two lines: the title (white/black, a touch
  bolder when selected) above its "#number • branch" (gray, fainter when
  deselected). Titles are fetched in one batched GraphQL query (PRTitles)
  and fall back to the branch name.
- Scroll long stacks in a fixed 10-item window with persistent "N more"
  indicators, so the list no longer jumps as those hints appear and
  disappear. Add shift+up / shift+down to jump to the top or bottom.

Progress and outcome:
- Always render a status line ("Submitting merge request...") so it does
  not pop in later and shift the view, and normalize messages to end in an
  ellipsis.
- Print the final result from the command layer rather than the TUI: a
  success line that includes the merge commit SHA
  ("Merged #1, #2 into main (abc1234)"), an atomic-rollback note on
  failure, a distinct message when the user stops watching an in-flight
  merge, and "Cancelled operation, nothing merged" on cancel.
- Clamp every rendered line to the terminal width so resizing no longer
  leaves duplicated header lines behind, and make truncation ANSI-aware.

Async-merge client:
- Use the go-gh REST client (c.rest.Put / c.rest.Get) for both the submit
  and poll endpoints, removing the bespoke http.Client, base-URL helper,
  and manual response decoding. The REST client discards non-2xx bodies,
  but that only costs the rare 400 message and 409 UUID: real merge
  failures still surface through the 200 poll body, and the in-range PRs
  are validated open, non-draft, and non-merged before submitting.
- Add classifyAsyncMergeError to map status codes to clear errors (404
  unavailable, 409 already exists, 400 no longer mergeable) and drop the
  now-unused AsyncMergeResult.StatusCode field. Rework the client tests to
  drive the REST client through a stub http.RoundTripper.

* warn merge queue unsupported

* update for new status field from api

* merge cmd docs

* more helpful error msgs

* update to support merge queue

* addressing review comments

* hide merge method step for merge queue

* set merge action explicitly

* address review comments to clarify docs on merge/api behavior

Author: skarim

.github/copilot-instructions.md

@@ -17,7 +17,7 @@ No Makefile, no code generation, no external linter config. Standard Go toolchai
 
 - `cmd/`: One Cobra command per file. Each exports `<Name>Cmd(cfg *config.Config)` with logic in `run<Name>()`.
 - `internal/git/`: `Ops` interface (52 methods) wrapping git CLI. `MockOps` for tests. Package-level functions delegate to swappable `ops` variable.
-- `internal/github/`: `ClientOps` interface (13 methods) for GitHub API. `MockClient` for tests. Stack operations use the public Stacks REST API (`/repos/{owner}/{repo}/stacks`).
+- `internal/github/`: `ClientOps` interface (18 methods) for GitHub API. `MockClient` for tests. Stack operations use the public Stacks REST API (`/repos/{owner}/{repo}/stacks`); merges use the async merge API (`/repos/{owner}/{repo}/pulls/{n}/merge-async`) with an explicit `merge_action` (`direct_merge` or `merge_queue`) chosen from the base branch's merge-queue detection. `merge_action` is optional — omitting it (or sending `default`) lets the server auto-route (merge queue if one is configured, else direct merge) — but the CLI sends it explicitly so a wrong detection fails loudly instead of silently merging directly.
 - `internal/config/`: `Config` struct passed to all commands. Holds I/O, colors, and test hooks (`SelectFn`, `ConfirmFn`, `InputFn`, `GitHubClientOverride`).
 - `internal/stack/`: Stack file (`.git/gh-stack`, JSON) management with file locking.
 - `internal/tui/`: bubbletea views (`stackview`, `modifyview`).

AGENTS.md

@@ -35,7 +35,7 @@ internal/
     gitops.go                # Ops interface (52 methods)
     mock_ops.go              # MockOps. Each method has a corresponding *Fn field.
   github/                    # github.ClientOps interface + real Client
-    client_interface.go      # ClientOps interface (13 methods)
+    client_interface.go      # ClientOps interface (18 methods)
     mock_client.go           # MockClient. Uses function-pointer fields for testing.
   stack/                     # stack file (.git/gh-stack) management, JSON schema, locking
     schema.json              # JSON Schema for the stack file format
@@ -57,7 +57,7 @@ skills/                      # AI agent skill definition (SKILL.md)
 | Group | Commands |
 |-------|----------|
 | Stack management | `init`, `add`, `view`, `checkout`, `modify`, `unstack` |
-| Remote operations | `submit`, `sync`, `rebase`, `push`, `link` |
+| Remote operations | `submit`, `sync`, `rebase`, `push`, `link`, `merge` |
 | Navigation | `switch`, `up`, `down`, `top`, `bottom`, `trunk` |
 | Utilities | `alias`, `feedback` |
 
@@ -109,7 +109,7 @@ if errors.As(err, &exitErr) { ... }
 ### Key interfaces
 
 - **`git.Ops`** (`internal/git/gitops.go`): 52 methods wrapping git CLI calls. The production implementation uses `cli/go-gh`'s `client.Command()` via `run()` and `runSilent()` helpers. Package-level functions (e.g., `git.CurrentBranch()`) delegate to a swappable package-level `ops` variable.
-- **`github.ClientOps`** (`internal/github/client_interface.go`): 13 methods for GitHub API (PRs, stacks). Stack operations use the public Stacks REST API (`/repos/{owner}/{repo}/stacks`): `ListStacks`, `FindStackForPR`, `GetStack`, `CreateStack`, `AddToStack` (delta append), `Unstack`. Injected via `cfg.GitHubClientOverride` in tests.
+- **`github.ClientOps`** (`internal/github/client_interface.go`): 18 methods for GitHub API (PRs, stacks, merges). Stack operations use the public Stacks REST API (`/repos/{owner}/{repo}/stacks`): `ListStacks`, `FindStackForPR`, `GetStack`, `CreateStack`, `AddToStack` (delta append), `Unstack`. Async stack merges use `RepoMergeConfig` (GraphQL: allowed merge methods + viewer's default), `BaseBranchUsesMergeQueue` (GraphQL: detects a base-branch merge queue to select the explicit `merge_action`), `MergeStackAsync`, and `GetAsyncMergeResult` (`/repos/{owner}/{repo}/pulls/{n}/merge-async`). Injected via `cfg.GitHubClientOverride` in tests.
 - **`config.Config`** (`internal/config/config.go`): Central configuration passed to all commands. Holds I/O streams, color functions, and test hook fields (`SelectFn`, `ConfirmFn`, `InputFn`, `RepoOverride`).
 
 ### Stack file

README.md

@@ -439,6 +439,46 @@ gh stack link 42 43 feature-auth feature-ui
 gh stack link --base develop --open feat-a feat-b feat-c
 ```
 
+### `gh stack merge`
+
+Merge one or multiple stacked PRs at once.
+
+```
+gh stack merge [<stack-number> | <pr-number>]
+```
+
+All members of the stack up to and including your chosen pull request are merged into the base branch in a single, all-or-nothing operation: if any PR can't be merged, none are.
+
+With no argument, the current active local stack is used. Pass a stack number to merge a stack you don't have checked out (a purely remote operation), or a pull request number to merge directly up to that PR.
+
+In an interactive terminal, a short wizard walks you through choosing which PRs to merge, picking the merge method, and confirming. In a non-interactive terminal, or with `--yes`, the whole stack (or everything up to the given PR) is merged without prompting, using your last-used merge method unless one is specified.
+
+Only basic pull request state is checked before merging (open and not a draft); GitHub evaluates branch protection and repository rules when the merge runs, so any such failure is reported back to you. **Bypassing merge requirements is not supported** for stacked PR merges.
+
+If the base branch uses a merge queue, the stack is added to the queue instead of merging directly. The queue chooses the merge method, so the wizard skips the method step and any `--merge-method` (or `--squash`/`--rebase`/`--merge`) flag is ignored with a warning. The selected pull requests are added to the queue together but merge as the queue processes them — they may land in separate groups rather than all at once.
+
+| Flag | Description |
+|------|-------------|
+| `--merge-method <method>` | Merge method to use: `merge`, `squash`, or `rebase` |
+| `--merge` / `--squash` / `--rebase` | Shorthands for the corresponding merge method |
+| `-y, --yes` | Merge without prompting for confirmation |
+
+**Examples:**
+
+```sh
+# Merge the current stack (interactive picker)
+gh stack merge
+
+# Merge a stack you don't have checked out, by stack number
+gh stack merge 7
+
+# Merge everything up to and including PR #42
+gh stack merge 42
+
+# Merge the whole current stack without prompting, squashing
+gh stack merge --yes --squash
+```
+
 ### `gh stack view`
 
 View the current stack.