If neither /etc/host-keys nor /root/host-keys has the target's SSH
host key, auto-install.sh previously went straight to "continue
without one anyway?". Added a third option in between, gated on
[[ -t 0 ]] (only offered when there's an actual operator at stdin, never
in an unattended/non-interactive run): prompt for an arbitrary
directory (USB stick, other mount, etc.), and if the key pair is
there, copy it into /root/host-keys and install it to /mnt same as the
existing pre-seeded-key path. Falls through to the original
warning+confirm if the prompt is skipped, the path doesn't have the
key, or the run isn't interactive at all.
docs/auto-installer.md updated to mention the new fallback. Quick
bash -n + shellcheck pass only, per request.