Give linode-* hosts a real Disko config, simplify auto-install.sh

Linode provisions and sizes /dev/sda (root) and /dev/sdb (swap) itself
as whole, unpartitioned block devices before the OS ever boots.
modules/disko/linode.nix declares them with destroy = false (skips
Disko's wipe stage for these disks entirely) and a bare filesystem/swap
content type matching that existing layout, so re-running it against an
already-provisioned disk only mkfs/mkswaps if blkid shows it isn't
formatted yet -- never repartitions or destroys data.

With every host reachable through the installer menu now carrying a
Disko config, auto-install.sh no longer needs to probe the flake and
branch between `disko --mode destroy,format,mount` and a bind-mount
fallback -- it just always runs Disko.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01La55Nsss8jZ7ZuzUV9mfot

modules/installer/common.nix

@@ -119,21 +119,12 @@
             nix_extra_opts+=(--option substituters "https://cache.nixos.org/")
         fi
 
-        if nix eval --refresh --json  --extra-experimental-features "flakes nix-command" "''${nix_extra_opts[@]}" "''${FLAKE_BASE_URL}#nixosConfigurations.''${choice}.config.disko.devices.disk.main.device" >/dev/null 2>&1; then
-            disko --mode destroy,format,mount \
-                --flake "''${FLAKE_BASE_URL}#''${choice}" "''${nix_extra_opts[@]}" --yes-wipe-all-disks
-        else
-            # No raw disk to partition — true for every LXC container, which has
-            # no block device visible from inside it, only its already-mounted
-            # root filesystem. nixos-install defaults to installing into /mnt; if
-            # nothing points /mnt at the real root, the new system gets built and
-            # written to a disconnected empty directory, never actually becomes
-            # bootable, and the container silently keeps running this installer
-            # environment forever. Bind-mount / onto /mnt so the install actually
-            # lands on the filesystem this container boots from.
-            echo "Selected host has no Disko configuration — installing in place via bind mount."
-            mount --bind / /mnt
-        fi
+        # Every host reachable through this menu has a Disko config (lxc-*
+        # is filtered out above, and is the only category that doesn't —
+        # see docs/auto-installer.md), so this can run unconditionally: no
+        # need to probe the flake first and branch on whether Disko applies.
+        disko --mode destroy,format,mount \
+            --flake "''${FLAKE_BASE_URL}#''${choice}" "''${nix_extra_opts[@]}" --yes-wipe-all-disks
 
         # sops-nix derives this host's decryption key from its own SSH host key
         # at *activation* time, which runs before systemd would otherwise