Fix LXC deployment path and clean up remaining eval warnings

LXC hosts (device busy fix):

modules/platforms/lxc.nix now imports nixpkgs' own
virtualisation/proxmox-lxc.nix, giving every lxc-* host a real
config.system.build.tarball output — a directly `pct restore`-able
Proxmox container image. This is the actual bug fix behind the
"cannot remove real root directory: device busy or in use" error:
lxc-* targets were only reachable through nixos-install, which
bind-mounts / onto /mnt for containers (no raw disk to partition)
and then correctly refuses to modify the filesystem it's currently
running on. auto-install.sh's menu now excludes lxc-* targets
entirely (they deploy via nix build + pct restore instead, see
docs/auto-installer.md) — and, on the same reasoning, also excludes
`installer`/`proxmox-lxc`, which are the installer image's own flake
targets, not deployable hosts.

manageHostName = true keeps host.nix's declared hostnames (upstream's
default would let Proxmox's ambient container config win instead);
privileged = false matches how these containers are actually created.

Eval warnings, now zero across all 19 nixosConfigurations + 4 packages:

- Multiple password options (root/nixos in the installer): nixpkgs'
  own installer profile sets initialHashedPassword = "" for
  passwordless login, conflicting with our explicit hashedPassword.
  Force-nulled the upstream option rather than adopting passwordless
  login, since this image now also boots over LAN PXE with
  PasswordAuthentication enabled.
- boot.zfs.forceImportRoot default value: set explicitly to false
  (matching the two places that already did) in
  modules/common/configuration.nix and modules/installer/common.nix,
  covering every host and the installer alike.
- Deprecated pkgs.system in modules/build-types/gui.nix: switched to
  pkgs.stdenv.hostPlatform.system.

All confirmed non-behavioral where it matters: unrelated hosts'
drvPaths are byte-identical to their pre-existing baselines throughout.

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

docs/auto-installer.md

@@ -12,6 +12,38 @@ discovers available hosts from this same flake, lets the operator choose a
 target, applies that host's Disko storage configuration, installs NixOS, and
 reboots.
 
+**This applies to every `nixosConfigurations` target except `lxc-*` hosts —
+see "LXC hosts" immediately below for why those are different.**
+
+## LXC hosts
+
+`lxc-*` targets (`lxc-minimal`, `lxc-nix-cache`, `lxc-server`, `lxc-docker`,
+`lxc-gui`, `lxc-pxe-boot`) are **not** installed via `auto-install.sh` — the
+interactive menu deliberately excludes them. Don't try to select one there;
+`nixos-install` would bind-mount `/` onto `/mnt` (LXC containers have no raw
+disk to partition) and then refuse to touch the filesystem it's currently
+running on — it's designed to protect exactly this case, so it just fails.
+
+`modules/platforms/lxc.nix` imports nixpkgs' own
+`virtualisation/proxmox-lxc.nix` module, which gives every `lxc-*` host a
+`config.system.build.tarball` output — a complete, directly Proxmox-importable
+container image, no install step at all:
+
+```sh
+nix build .#nixosConfigurations.lxc-minimal.config.system.build.tarball
+```
+
+Then, on the Proxmox host, `pct restore` (or the GUI's "Create CT" → upload
+template flow) that tarball directly as a new container. First boot runs
+`boot.postBootCommands` (registers the Nix store DB and system profile) —
+there's no separate activation step to run yourself.
+
+Host keys still need pre-seeding the same way as any other host (see "Host
+keys" below) — the sops-nix activation-vs-first-boot race is identical
+regardless of how the image reaches the machine. `NIXOS_HOST_KEYS_DIR=...
+nix build ... --impure` bakes the matching key into the tarball the same way
+it does for the ISO/PXE installer images.
+
 ## Layout
 
 - `modules/installer/common.nix` — shared by every installer target: SSH