Add baremetal-gui flake target with ZFS RAID0, AMD GPU, and sops-backed wifi

Wires everything staged so far into a real flake target:

- modules/platforms/baremetal.nix (new): the bare-metal platform module,
  composed from a real nixos-generate-config run on the actual gui-host
  hardware (AMD CPU, ahci/xhci/usb storage -- modules/hardware-configuration/baremetal.nix).
  Enables hardware.enableRedistributableFirmware (real wifi/GPU/microcode
  firmware VMs never needed), amdgpu as the Xorg video driver plus
  hardware.graphics for Mesa OpenGL/Vulkan, and imports the ZFS RAID0 disko
  layout + modules/services/zfs/enable-service.nix for root-on-ZFS boot
  support.
- flake.nix: new baremetal-gui target, reusing hosts/nixos/host.nix (same
  identity already shared across linode/proxmox/lxc-gui).
- hosts/nixos/host.nix: added networking.hostId, required now that a ZFS
  root pool is in the picture.
- variables.nix: guiRootDisk1/guiRootDisk2 filled in (/dev/sda, /dev/sdb --
  only used transiently at disko-format time, same as modules/disko/proxmox.nix's
  own plain device path). wifiPassword removed.
- modules/networking/wifi.nix: reworked to pull the wifi password from a
  new sops secret (secrets/gui.yaml, wifi-password) instead of a plaintext
  variable -- NetworkManager's ensureProfiles renders `psk = "$WIFI_PASSWORD"`
  literally (nixpkgs' own documented pattern for this) and envsubst-expands
  it from a sops-rendered EnvironmentFile at activation, so the real value
  never touches the Nix store, only /run.
- .sops.yaml: new secrets/gui\.yaml rule, admin + the currently-registered
  lxc-gui recipient (the only gui variant with a provisioned host key so
  far -- whichever variant is actually deployed next still needs
  scripts/secrets/sync-host-keys.sh run for its own recipient).
- README.md/CLAUDE.md: documented the new platform/target and its module
  layout, per this repo's own drift-prevention note.

Verified end-to-end: nix eval of every existing target (nothing broke),
a temporary real nixosSystem build against the actual disko.nixosModules.disko
confirming the generated zpool create has no mirror/raidz keyword (genuine
stripe), and a temporary test SSID confirming the sops secret/template/
ensureProfiles chain renders correctly before reverting to blank/real values.
Full scripts/codex-maintenance.sh (secret-grep, fmt, statix, full-fallback
eval of every host/package) passes clean.

flake.nix

@@ -91,6 +91,7 @@
         linode-gui = mkTarget { platform = "linode"; buildType = "gui"; hostPath = ./hosts/nixos/host.nix; homeFile = ./hosts/nixos/home.nix; };
         proxmox-gui = mkTarget { platform = "proxmox"; buildType = "gui"; hostPath = ./hosts/nixos/host.nix; homeFile = ./hosts/nixos/home.nix; };
         lxc-gui = mkTarget { platform = "lxc"; buildType = "gui"; hostPath = ./hosts/nixos/host.nix; homeFile = ./hosts/nixos/home.nix; };
+        baremetal-gui = mkTarget { platform = "baremetal"; buildType = "gui"; hostPath = ./hosts/nixos/host.nix; homeFile = ./hosts/nixos/home.nix; };
 
         proxmox-pxe-boot = mkTarget { platform = "proxmox"; buildType = "pxe-boot"; hostPath = ./hosts/pxe-boot/host.nix; };
         lxc-pxe-boot = mkTarget { platform = "lxc"; buildType = "pxe-boot"; hostPath = ./hosts/pxe-boot/host.nix; };