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>

modules/common/aliases.nix

@@ -1,19 +1,24 @@
 { config, pkgs, lib, ... }:
 
 let
+  # Flake attribute names are now <platform>-<buildtype> (e.g. proxmox-docker)
+  # and no longer match networking.hostName, since a host's hostname stays
+  # fixed while the platform backing it can change. Each nixosConfiguration
+  # stamps its own active target name into /etc/flake-target at build time.
   mySwitchCmd = ''
     sudo nixos-rebuild switch \
       --no-write-lock-file \
       --refresh \
-      --flake git+https://gitea.lan.ddnsgeek.com/beatzaplenty/nixos.git#$(hostname)
+      --flake git+https://gitea.lan.ddnsgeek.com/beatzaplenty/nixos.git#$(cat /etc/flake-target)
   '';
   myTestCmd = ''
     sudo nixos-rebuild test \
       --no-write-lock-file \
       --refresh \
-      --flake git+https://gitea.lan.ddnsgeek.com/beatzaplenty/nixos.git#$(hostname)
+      --flake git+https://gitea.lan.ddnsgeek.com/beatzaplenty/nixos.git#$(cat /etc/flake-target)
   '';
-in {
+in
+{
   programs.bash = {
     enable = true;
     shellAliases = {