fix(ssh): brace refspec variables for zsh login shells (#3383)

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>
Co-authored-by: Carlos Florencio <carlosmflorencio@gmail.com>

apps/backend/internal/agent/runtime/lifecycle/default_scripts.go

@@ -298,7 +298,7 @@ if [ -n "$repository_url" ]; then
     echo 'kandev: target repository has no base branch' >&2
     exit 1
   fi
-  if ! git -C "$workspace" fetch --no-tags origin "+refs/heads/$repository_branch:refs/remotes/origin/$repository_branch" >/dev/null 2>&1; then
+  if ! git -C "$workspace" fetch --no-tags origin "+refs/heads/${repository_branch}:refs/remotes/origin/${repository_branch}" >/dev/null 2>&1; then
     echo 'kandev: target base branch is unavailable' >&2
     exit 1
   fi