Simplify installer into shared common.nix + per-platform targets

Replaces the single monolithic installer.nix (preserved as
installer_old.nix for reference) with a shared common.nix carrying the
install-script/user/SSH baseline, consumed by per-platform targets
(installer.nix for netboot/ISO, new proxmox-lxc.nix for the Proxmox
LXC-based flow). flake.nix drops the nixos-generators input in favor
of building images directly from each nixosConfiguration's
system.build.isoImage/tarball.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

proxmox-lxc.nix

@@ -0,0 +1,32 @@
+{ pkgs, ... }:
+
+{
+  imports = [
+    ./common.nix
+  ];
+
+
+  # LXC has no bootloader
+  boot.loader.grub.enable = false;
+  boot.loader.systemd-boot.enable = false;
+
+
+  # Ensure Proxmox console works
+#   services.getty.enable = true;
+
+
+#   environment.etc."auto-install.sh" = {
+#     source = ./auto-install.sh;
+#     mode = "0755";
+#   };
+
+
+  # Run installer when logging in
+  environment.etc."bash_profile-nixos".text = ''
+    if [ -n "$PS1" ] && [ ! -e ~/.auto_install_ran ]; then
+      sudo /etc/auto-install.sh
+      touch ~/.auto_install_ran
+    fi
+  '';
+# networking.useDHCP = true;
+}