feat(rpi5): add placeholder hardware-configuration

Committed so the lyrathorpe-rpi5 host evaluates in CI before the Pi is
provisioned. It is a placeholder, not a bootable config: on first install,
regenerate it on the device with nixos-generate-config and replace this file.
Excluded from formatters/linters by the existing hardware-configuration.nix
rules.

system/machine/RPi5/hardware-configuration.nix

@@ -0,0 +1,31 @@
+# PLACEHOLDER hardware configuration for the Raspberry Pi 5.
+#
+# This file is NOT the real generated config -- it exists only so the host
+# evaluates in CI before the Pi is provisioned. The machine will not boot from
+# it as-is. On first install, regenerate this file on the device with
+#   nixos-generate-config --root /mnt
+# and replace this placeholder with the output (commit it). See ./README.md.
+#
+# Like every hardware-configuration.nix in this repo, this file is excluded from
+# the formatter and linters (see the pre-commit/treefmt excludes in flake.nix).
+{ modulesPath, ... }:
+{
+  imports = [ (modulesPath + "/installer/scan/not-detected.nix") ];
+
+  nixpkgs.hostPlatform = "aarch64-linux";
+
+  # The Raspberry Pi 5 boots from an SD card / USB with a FAT firmware partition
+  # and an ext4 root. Labels match the conventional sd-image layout; the real
+  # generated config will use by-uuid device paths instead.
+  fileSystems."/" = {
+    device = "/dev/disk/by-label/NIXOS_SD";
+    fsType = "ext4";
+  };
+
+  fileSystems."/boot/firmware" = {
+    device = "/dev/disk/by-label/FIRMWARE";
+    fsType = "vfat";
+  };
+
+  swapDevices = [ ];
+}