Restructure flake into per-host modules

Move host-specific files under hosts/<name>/ so a second machine can be
added without forking the repo. Split system/users.nix and
system/display-manager.nix into per-option subfolders, separate the
home-manager packages list into a shared base and a personal bundle,
extract networking.hostName out of the shared networking module, and
inline the git identity into the per-host home.nix. Add .gitignore
guardrails to keep secrets out.

hosts/hivebrain/default.nix

@@ -0,0 +1,26 @@
+{ ... }:
+
+{
+  imports = [
+    ./hardware-configuration.nix
+
+    # shared
+    ../../system/boot.nix
+    ../../system/networking.nix
+    ../../system/locale.nix
+    ../../system/fonts.nix
+    ../../system/nix.nix
+    ../../system/ssh.nix
+    ../../system/programs.nix
+    ../../system/packages.nix
+
+    # marvin-only
+    ../../system/xserver.nix
+    ../../system/picom.nix
+    ../../system/display-managers/ly.nix
+    ../../system/users/marvin.nix
+  ];
+
+  networking.hostName = "hivebrain";
+  system.stateVersion = "26.05";
+}

hosts/hivebrain/hardware-configuration.nix

@@ -0,0 +1,36 @@
+# Do not modify this file!  It was generated by ‘nixos-generate-config’
+# and may be overwritten by future invocations.  Please make changes
+# to /etc/nixos/configuration.nix instead.
+{ config, lib, pkgs, modulesPath, ... }:
+
+{
+  imports =
+    [ (modulesPath + "/installer/scan/not-detected.nix")
+    ];
+
+  boot.initrd.availableKernelModules = [ "xhci_pci" "ahci" "nvme" "usb_storage" "usbhid" "sd_mod" ];
+  boot.initrd.kernelModules = [ ];
+  boot.kernelModules = [ "kvm-amd" ];
+  boot.extraModulePackages = [ ];
+
+  hardware.graphics.enable = true;
+  hardware.graphics.enable32Bit = true;
+
+  fileSystems."/" =
+    { device = "/dev/disk/by-uuid/e9f91ba2-1d73-47ed-a37e-b847a9723b0f";
+      fsType = "ext4";
+    };
+
+  fileSystems."/boot" =
+    { device = "/dev/disk/by-uuid/776C-43F7";
+      fsType = "vfat";
+      options = [ "fmask=0022" "dmask=0022" ];
+    };
+
+  swapDevices =
+    [ { device = "/dev/disk/by-uuid/d4f6670c-97e8-4297-aeec-99ade922f133"; }
+    ];
+
+  nixpkgs.hostPlatform = lib.mkDefault "x86_64-linux";
+  hardware.cpu.amd.updateMicrocode = lib.mkDefault config.hardware.enableRedistributableFirmware;
+}

hosts/hivebrain/home.nix

@@ -0,0 +1,25 @@
+{ ... }:
+
+{
+  imports = [
+    ../../home/zsh.nix
+    ../../home/fonts.nix
+    ../../home/rofi.nix
+    ../../home/yazi.nix
+    ../../home/symlinks.nix
+    ../../home/packages-base.nix
+    ../../home/packages-marvin.nix
+  ];
+
+  home.username = "marvin";
+  home.homeDirectory = "/home/marvin";
+  home.stateVersion = "26.05";
+
+  programs.git = {
+    enable = true;
+    settings.user = {
+      name = "MDietermann";
+      email = "marvin.dietermann@hive-brain.de";
+    };
+  };
+}