Unmount everything under /mnt before zpool export, not just the chroot dirs

The previous fix (#42) only unmounted /mnt/{dev,proc,sys,run}, but disko
also mounts the ESP at /mnt/boot (modules/disko/baremetal.nix) -- another
nested mount blocking ZFS from unmounting its own root dataset at /mnt
the same way. Confirmed live: zpool export still failed with "cannot
unmount '/mnt': pool or dataset busy" after the chroot-only fix.

Replace the manual dev/proc/sys/run list with a single recursive
`umount -R /mnt`, which clears everything nested under /mnt -- current
and future mountpoints alike -- rather than needing to keep enumerating
whatever nixos-install/disko happen to leave mounted.

modules/platforms/baremetal.nix

@@ -25,4 +25,14 @@
     enable = true;
     enable32Bit = true;
   };
+
+  # The systemd-based initrd (default here since this host has a ZFS root --
+  # see modules/disko/baremetal.nix) locks the root account by default, so
+  # sulogin refuses to hand over a shell if something in the initrd (e.g.
+  # the ZFS pool import) fails and it drops to emergency mode -- confirmed
+  # live: it just loops re-entering the target instead of prompting. This
+  # only affects the pre-switch-root initrd shell, not the installed
+  # system's own login, and is worth the tradeoff on a box already reachable
+  # at the physical console.
+  boot.initrd.systemd.emergencyAccess = true;
 }

scripts/installer/auto-install.sh

@@ -179,6 +179,24 @@ nixos-install \
     # makes boot stall waiting on the ZFS import. Exporting here (a no-op
     # if the chosen host has no ZFS root, e.g. proxmox-*/linode-*) clears
     # that in-use state so the next import, from any hostid, succeeds.
+    #
+    # Anything still mounted under /mnt -- nixos-install's own leftover
+    # chroot bind mounts for running the target's activation script
+    # (/mnt/dev, /mnt/proc, /mnt/sys, /mnt/run), and disko's own /mnt/boot
+    # ESP mount (modules/disko/baremetal.nix) -- blocks ZFS from unmounting
+    # its root dataset at /mnt, the same way any nested mount blocks
+    # unmounting its parent. Confirmed live: zpool export failed with
+    # "cannot unmount '/mnt': pool or dataset busy" even after handling the
+    # chroot mounts alone, because /mnt/boot was still mounted too. Because
+    # of this script's `set -e`, that killed the script before it ever
+    # reached reboot, silently defeating the whole point of exporting first.
+    # Unmounting everything under /mnt up front (recursively, so nested
+    # mounts like /mnt/dev/pts come along for free) sidesteps needing to
+    # enumerate every mount disko/nixos-install might leave behind.
+    if mountpoint -q /mnt; then
+        umount -R /mnt
+    fi
+
     if [[ -n "$(zpool list -H -o name 2>/dev/null)" ]]; then
         echo "Exporting ZFS pool(s) before reboot..."
         zpool export -a