`hermes update` fails when run from a non-main branch. The update logic falls back to pulling `origin/main`, but uses `git pull --ff-only origin main` which can't fast-forward into a diverged feature branch.

Repro: check out any feature branch with local commits, run `hermes update`.

fatal: Not possible to fast-forward, aborting.
✗ Update failed: Command '['git', 'pull', '--ff-only', 'origin', 'main']' returned non-zero exit status 128.

Expected: update should succeed regardless of which branch the user is on.