fix: resolve default branch explicitly when updating shallow-cloned custom modules (#2332)

With shallow clones (--depth 1), `origin/HEAD` becomes stale after the
initial clone. The update path used `git reset --hard origin/HEAD` which
never picked up new commits pushed to the default branch.

Resolve the default branch name via `git symbolic-ref refs/remotes/origin/HEAD`,
then fetch and reset against `origin/<branch>` explicitly. Falls back to
`main` if origin/HEAD is not set.

Co-authored-by: Brian <bmadcode@gmail.com>