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-cache/host.nix

@@ -0,0 +1,16 @@
+{ ... }:
+
+{
+  networking.hostName = "nix-cache";
+
+  services.beszel.agent.environment = {
+    #DOCKER_HOST = "tcp://docker-socket-proxy:2375";
+    #HUB_URL = "http://docker.sweet.home:8090";
+    KEY = "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIFPR9kwtC4TAeTRu46A7+opZsYpxqkRJ+x/ZyB2GWCeG";
+    TOKEN = "c9192e4c-7b5d-4910-8241-c2d68afadbac";
+  };
+
+  # Preserved from the pre-refactor `nix-cache` target — stateVersion must
+  # never be bumped on an already-installed machine.
+  system.stateVersion = "25.05";
+}