Add Proxmox VM disk-image building; fix disko confirmation bypass

modules/disko/proxmox.nix gains imageSize (20G default) and a
per-host imageName (networking.hostName, so every proxmox-* host
produces a distinctly named image instead of an identical main.raw).
This is the same disko.devices config already used to format a real
disk on install, so it's available for every proxmox-* target with no
per-host changes needed:

  nix build .#nixosConfigurations.<host>.config.system.build.diskoImagesScript
  sudo ./result --build-memory 2048

docs/proxmox-images.md covers building, host-key pre-seeding via
disko's --pre-format-files (same host-keys/ workflow as the installer
and LXC tarball paths), and the qm import/attach sequence for
deploying the result to Proxmox.

Also fixes a real bug in auto-install.sh found while testing: the
disko confirmation bypass used --yes, which disko's CLI doesn't
recognize at all (the actual flag is --yes-wipe-all-disks) — so the
"skip confirmation" flag was silently a no-op and the interactive
prompt kept appearing regardless.

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

modules/installer/common.nix

@@ -123,7 +123,7 @@
 
         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
+                --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