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>

.github/workflows/check-nixos.yml

@@ -20,7 +20,9 @@ jobs:
       - name: Evaluate all NixOS hosts
         run: |
           set -euo pipefail
-          for host in nixos docker server nix-cache nix-minimal pxe-boot linode-minimal; do
+          hosts="$(nix --extra-experimental-features 'nix-command flakes' eval --json \
+            .#nixosConfigurations --apply builtins.attrNames | jq -r '.[]')"
+          for host in $hosts; do
             echo "Evaluating ${host}"
             nix --extra-experimental-features 'nix-command flakes' eval \
               ".#nixosConfigurations.${host}.config.system.build.toplevel.drvPath" --raw