Issue description
MyUX repository 55 was hosted initially with default branch qt6. akurai_repo_update successfully changed metadata default_branch to main, and main was pushed, but the hosted bare repository HEAD still targets refs/heads/qt6. Git correctly refuses deletion of qt6 as the current branch. akurai_repo_sync does not repair HEAD. Acceptance: updating the default branch on a trusted hosted repository atomically validates the target ref, updates repository metadata and the bare symbolic HEAD, and permits deletion of the previous default branch without direct filesystem mutation.