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/nix-minimal/configuration.nix

@@ -1,50 +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.
-      # inputs.disko.nixosModules.disko
-      ../../modules/common/configuration.nix
-      ../../modules/nix-cache/client.nix
-      ../../modules/remote-builder-client.nix
-      ../../modules/disko/proxmox.nix
-      ../../modules/boot/efi.nix
-    ];
-
-  networking.hostName = "nix-minimal"; # Define your hostname.
-
-  
-  # Open ports in the firewall.
-#  networking.firewall.allowedTCPPorts = [ 80 8080 443 ];
-  # 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.sta>
-  system.stateVersion = "25.05"; # Did you read the comment?
-}