GitLab mr checkout: branch and force handling is unreachable #228

GitLabCli.checkoutMergeRequest adds --branch and --force to glab mr checkout when its input sets branch or force (packages/source-control-gitlab/src/server/GitLabCli.ts, the Coder: notes on the input type and in checkoutMergeRequest).

No caller passes either field. The only path is GitManager → checkoutChangeRequest({ cwd, reference }) (apps/server/src/git/GitManager.ts), the same as upstream. So that part of the fork's change never runs and conflicts with upstream for nothing.

Fix: drop the branch field and the --branch and --force handling, and update the GitLab seam entry in docs/internals/coder-only.md. Keep the --repo <url> … -- <iid> selector, which is reachable.