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/disko/linode.nix

@@ -0,0 +1,35 @@
+_:
+
+{
+  # Linode provisions and sizes these disks itself (via the Linode
+  # dashboard/API) before the OS ever boots, and presents them as whole,
+  # unpartitioned block devices — /dev/sda is the root filesystem directly,
+  # /dev/sdb is swap directly, no partition table on either. Nothing here
+  # should ever repartition or resize them:
+  #   - `destroy = false` skips each disk entirely during disko's destroy
+  #     stage (see disko's disk.destroy option) — no wipefs, ever.
+  #   - the filesystem content type's own create step only runs mkfs if the
+  #     device isn't already formatted (checked via `blkid`), so re-running
+  #     this against an already-provisioned Linode disk is a no-op.
+  disko.devices.disk = {
+    main = {
+      device = "/dev/sda";
+      destroy = false;
+
+      content = {
+        type = "filesystem";
+        format = "ext4";
+        mountpoint = "/";
+      };
+    };
+
+    swap = {
+      device = "/dev/sdb";
+      destroy = false;
+
+      content = {
+        type = "swap";
+      };
+    };
+  };
+}