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/hardware-configuration/vm/linode.nix

@@ -33,14 +33,11 @@
     };
   };
 
-  fileSystems."/" =
-    {
-      device = "/dev/sda";
-      fsType = "ext4";
-    };
-
-  swapDevices =
-    [{ device = "/dev/sdb"; }];
+  # fileSystems."/" and swapDevices are now owned by disko
+  # (../disko/linode.nix, imported from ../platforms/linode.nix) — same
+  # /dev/sda root + /dev/sdb swap layout, declared there instead so disko's
+  # (idempotent, non-destructive — see that file) format/mount scripts stay
+  # in sync with what NixOS actually mounts.
 
   nixpkgs.hostPlatform = lib.mkDefault "x86_64-linux";
 }