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>

AGENTS.md

@@ -5,15 +5,13 @@
 This repository contains flake-based NixOS configurations for Wayne's LAN
 servers and workstation.
 
-The flake currently exposes these NixOS configurations:
-
-- `nixos`
-- `docker`
-- `server`
-- `nix-cache`
-- `nix-minimal`
-- `pxe-boot`
-- `linode-minimal`
+The flake exposes NixOS configurations named `<platform>-<buildtype>`
+(platforms: `linode`, `proxmox`, `lxc`; build types: `minimal`, `nix-cache`,
+`server`, `docker`, `gui`, `pxe-boot`), generated from `modules/platforms/*`
+and `modules/build-types/*` by the `mkTarget` function in `flake.nix`. Not
+every combination is built — `pxe-boot` has no `linode` variant. See
+`README.md` for the full current target list; treat `flake.nix` as the
+source of truth since this list can drift.
 
 Do not deploy, switch, reboot, repartition, format disks, or run destructive
 install commands from this repository unless explicitly asked.