Refactor flake targets into platform x build-type matrix

Generates all nixosConfigurations from mkTarget(platform, buildType,
hostPath) instead of hand-written per-host blocks, so adding a new
platform or build type is a one-line addition. Per-machine identity
(hostname, hostId, secrets, stateVersion) moves into hosts/<name>/host.nix;
platform-specific config (hardware, boot, networking) into
modules/platforms/*.nix; build-type config (minimal/server/docker/gui/
nix-cache/pxe-boot) into modules/build-types/*.nix.

Old flat targets (nixos, docker, server, nix-cache, nix-minimal, pxe-boot)
are replaced by the 17-target <platform>-<buildtype> matrix; each new
target was verified to evaluate before its old counterpart was removed.
CI workflows and docs/aliases now discover hosts dynamically via
nixosConfigurations attrNames and /etc/flake-target instead of hardcoded
lists, so they can't drift from flake.nix again.

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

hosts/linode-minimal/configuration.nix

@@ -1,144 +0,0 @@
-# Edit this configuration file to define what should be installed on
-# your system. Help is available in the configuration.nix(5) man page, on
-# https://search.nixos.org/options and in the NixOS manual (`nixos-help`).
-
-{ config, lib, pkgs, ... }:
-
-{
-  imports =
-    [ # Include the results of the hardware scan.
-      ../../modules/common/configuration.nix
-    ];
-
-  # Use the GRUB 2 boot loader.
-  boot.loader.grub.enable = true;
-  # boot.loader.grub.efiSupport = true;
-  # boot.loader.grub.efiInstallAsRemovable = true;
-  # boot.loader.efi.efiSysMountPoint = "/boot/efi";
-  # Define on which hard drive you want to install Grub.
-  boot.loader.grub.device = "/dev/sda"; # or "nodev" for efi only
-
-  # networking.hostName = "nixos"; # Define your hostname.
-
-  # Configure network connections interactively with nmcli or nmtui.
-  networking.networkmanager.enable = true;
-
-  # Set your time zone.
-  # time.timeZone = "Europe/Amsterdam";
-
-  # Configure network proxy if necessary
-  # networking.proxy.default = "http://user:password@proxy:port/";
-  # networking.proxy.noProxy = "127.0.0.1,localhost,internal.domain";
-
-  # Select internationalisation properties.
-  # i18n.defaultLocale = "en_US.UTF-8";
-  # console = {
-  #   font = "Lat2-Terminus16";
-  #   keyMap = "us";
-  #   useXkbConfig = true; # use xkb.options in tty.
-  # };
-
-  # Enable the X11 windowing system.
-  # services.xserver.enable = true;
-
-
-
-
-  # Configure keymap in X11
-  # services.xserver.xkb.layout = "us";
-  # services.xserver.xkb.options = "eurosign:e,caps:escape";
-
-  # Enable CUPS to print documents.
-  # services.printing.enable = true;
-
-  # Enable sound.
-  # services.pulseaudio.enable = true;
-  # OR
-  # services.pipewire = {
-  #   enable = true;
-  #   pulse.enable = true;
-  # };
-
-  # Enable touchpad support (enabled default in most desktopManager).
-  # services.libinput.enable = true;
-
-  # Define a user account. Don't forget to set a password with ‘passwd’.
-  # users.users.alice = {
-  #   isNormalUser = true;
-  #   extraGroups = [ "wheel" ]; # Enable ‘sudo’ for the user.
-  #   packages = with pkgs; [
-  #     tree
-  #   ];
-  # };
-
-  # programs.firefox.enable = true;
-
-  # List packages installed in system profile.
-  # You can use https://search.nixos.org/ to find more packages (and options).
-  # environment.systemPackages = with pkgs; [
-  #   vim # Do not forget to add an editor to edit configuration.nix! The Nano editor is also installed by default.
-  #   wget
-  # ];
-
-  # Some programs need SUID wrappers, can be configured further or are
-  # started in user sessions.
-  # programs.mtr.enable = true;
-  # programs.gnupg.agent = {
-  #   enable = true;
-  #   enableSSHSupport = true;
-  # };
-
-environment.systemPackages = with pkgs; [
-  inetutils
-  mtr
-  sysstat
-];
-
-users.users.nixos = {
-  isNormalUser = true;
-  home = "/home/nixos";
-  description = "nixos";
-  extraGroups = [ "wheel" "networkmanager" ];
-};
-
-networking.usePredictableInterfaceNames = false;
-networking.useDHCP = false;
-networking.interfaces.eth0.useDHCP = true;
-
-  # Enable the OpenSSH daemon.
-   services.openssh = {
-    enable = true;
-#    settings.permitRootLogin = "yes";
-};
-  # Open ports in the firewall.
-  # networking.firewall.allowedTCPPorts = [ ... ];
-  # networking.firewall.allowedUDPPorts = [ ... ];
-  # Or disable the firewall altogether.
-  # networking.firewall.enable = false;
-
-  # Copy the NixOS configuration file and link it from the resulting system
-  # (/run/current-system/configuration.nix). This is useful in case you
-  # accidentally delete configuration.nix.
-  # system.copySystemConfiguration = true;
-
-  # This option defines the first version of NixOS you have installed on this particular machine,
-  # and is used to maintain compatibility with application data (e.g. databases) created on older NixOS versions.
-  #
-  # Most users should NEVER change this value after the initial install, for any reason,
-  # even if you've upgraded your system to a new NixOS release.
-  #
-  # This value does NOT affect the Nixpkgs version your packages and OS are pulled from,
-  # so changing it will NOT upgrade your system - see https://nixos.org/manual/nixos/stable/#sec-upgrading for how
-  # to actually do that.
-  #
-  # This value being lower than the current NixOS release does NOT mean your system is
-  # out of date, out of support, or vulnerable.
-  #
-  # Do NOT change this value unless you have manually inspected all the changes it would make to your configuration,
-  # and migrated your data accordingly.
-  #
-  # For more information, see `man configuration.nix` or https://nixos.org/manual/nixos/stable/options#opt-system.stateVersion .
-  system.stateVersion = "26.05"; # Did you read the comment?
-
-}
-

hosts/linode-minimal/host.nix

@@ -0,0 +1,9 @@
+{ ... }:
+
+{
+  networking.hostName = "linode-minimal";
+
+  # Preserved from the pre-refactor `linode-minimal` target — stateVersion
+  # must never be bumped on an already-installed machine.
+  system.stateVersion = "26.05";
+}