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

README.md

@@ -93,7 +93,10 @@ Three different paths depending on target, none of them involving a manual
 `nixos-rebuild switch` from this repo:
 
 - Most hosts: boot the auto-installer, pick the target from its menu — see
-  `docs/auto-installer.md`.
+  `docs/auto-installer.md`. Every menu target has a Disko config the
+  installer formats unconditionally (`docs/auto-installer.md`'s "Storage"
+  section covers how this stays non-destructive for `linode-*`, whose disks
+  Linode itself provisions ahead of time).
 - `lxc-*` targets: not installed at all — build a ready-to-run container
   tarball and `pct restore` it directly. `docs/auto-installer.md` covers why
   (and the installer's menu excludes them for the same reason).