`stage_repository` treated a failed `pull --ff-only` as "keep local state"
and carried on. Every later stage reads files only the new tree has (`pm/`),
so an install whose checkout could not fast-forward failed further down with
`ModuleNotFoundError: No module named 'pm'` instead of updating.
A release cut off the main line diverges for every user who installed from it:
v2026.5.29.2 is not an ancestor of main, so its checkout can never fast-forward
to a later main. Match the remote the way `hermes update` already does, after
parking the old tip behind refs/hermes-install-backup/<stamp>-<sha> and
stashing any local work so neither is destroyed silently.
Verified against a real diverged repository: before, the checkout stayed on
the old line (HEAD=6a2e091, remote=43eea63) with no backup; after, HEAD equals
the new main, the backup ref and an autostash exist, and both are named in the
installer's output.
5cb0a5073b