fix(pulls): report stack merge API permission errors

Assisted-by: Codet:GPT-5-Codex

modules/task/pull_stack.go

@@ -120,7 +120,7 @@ func AddPullStackEntry(ctx stdctx.Context, login *config.Login, owner, repo stri
 	return &s, nil
 }
 
-// RestackPullStack queues a restack of every entry above the merged pull request in the stack.
+// RestackPullStack queues a restack starting at the stack's base branch.
 func RestackPullStack(ctx stdctx.Context, login *config.Login, owner, repo string, id int64) error {
 	client := api.NewClient(login)
 	resp, err := client.DoContext(ctx, http.MethodPost, fmt.Sprintf("/repos/%s/%s/stacks/%d/restack", owner, repo, id), nil, nil)
@@ -156,12 +156,18 @@ func MergePullStack(ctx stdctx.Context, login *config.Login, owner, repo string,
 	if resp.StatusCode == http.StatusAccepted {
 		return nil, true, nil
 	}
+	if resp.StatusCode != http.StatusOK && resp.StatusCode != http.StatusConflict {
+		return nil, false, stackAPIError(resp)
+	}
 
 	var body stack.MergeResult
 	if decodeErr := json.NewDecoder(resp.Body).Decode(&body); decodeErr != nil {
 		return nil, false, fmt.Errorf("%s: failed to decode response: %w", resp.Status, decodeErr)
 	}
 	if resp.StatusCode != http.StatusOK {
+		if body.Error == "" {
+			return &body, false, fmt.Errorf("%s", resp.Status)
+		}
 		return &body, false, fmt.Errorf("%s", body.Error)
 	}
 	return &body, false, nil