[Bug]: GitLab Merge button fails: glab mr merge is run without --sha ("SHA must be provided when merging") #17007

Before submitting

• I searched existing issues and did not find a duplicate.
• I included enough detail to reproduce or investigate the problem.

Area

apps/server

Steps to reproduce

1. Open a thread whose project lives on a gitlab.com project that rejects merges without a head SHA (the API answers 400 SHA must be provided when merging).
2. Have an open, mergeable merge request (detailed_merge_status: mergeable, pipeline passed, user.can_merge: true, Maintainer role).
3. Press Merge in the pull request panel.

Expected behavior

The merge request is merged.

Actual behavior

The action fails with the generic toast:

The host refused the merge. Check that you have write access, that the checks it requires have passed, and that the branch is not conflicting.

The server runs (apps/server/src/pullRequest/GitLabPullRequestCli.ts, actionArgs / runMergeRequestAction):

glab mr merge <iid> --repo <group/project> --auto-merge=false --yes

No --sha is passed, and GitLab rejects the request. Running the same command by hand shows the real error:

ERROR
All attempts fail:
#1: PUT https://gitlab.com/api/v4/projects/<group>%2F<project>/merge_requests/<iid>/merge:
400 {message: SHA must be provided when merging}.

Adding --sha <head sha> to the same command merges successfully.

Two problems:

1. GitLab merges don't send the head SHA. glab mr merge supports --sha, and the server already knows the MR head it displayed. Passing it fixes projects that require it, and also guarantees that only the commit the user reviewed gets merged (GitLab answers 409 if the branch moved). The same applies to enable-auto-merge.
2. glab's stderr is dropped. The failure surfaces as GitLabCliCommandError: GitLab CLI command failed. in the trace and as the generic toast in the UI, so the actual reason (SHA must be provided when merging) is invisible. It looked like an authentication problem until reproduced by hand.

Impact

Blocks work completely

Version or commit

0.0.46-nightly.20261007.2774 (code unchanged on main @ 3143335)

Environment

T3 server on Fedora Linux arm64 (headless t3 serve), T3 Desktop on macOS as client; glab 1.121.0 authenticated via token (glab auth status OK); gitlab.com.

Logs or stack traces

VcsProcessExitError: VCS process failed in GitLabCli.execute: glab (<repo>) exited with 1 - Process exited with a non-zero status.
PullRequestOperationError: Pull request operation runAction failed: GitLab CLI command failed.

Workaround

Merge from the GitLab web UI, or run glab mr merge <iid> --auto-merge=false --yes --sha <head sha> manually.