Prompt for a host-key path interactively as a third fallback

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.

docs/auto-installer.md

@@ -195,6 +195,10 @@ default.
 `auto-install.sh` still supports the older manual path as a fallback: if a
 host's key isn't baked in (`/etc/host-keys`), it checks `/root/host-keys`
 next, where you can `scp` a key in after boot, same as before this migration.
+If neither has it and the script is running interactively (an actual
+operator at the other end of stdin, not an unattended run), it prompts for
+an arbitrary directory to check (a mounted USB stick, another filesystem,
+etc.) and copies the key pair into `/root/host-keys` from there if found.
 
 ## Storage