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.

home/git.nix

@@ -1,11 +0,0 @@
-{ ... }:
-
-{
-  programs.git = {
-    enable = true;
-    settings.user = {
-      name = "MDietermann";
-      email = "marvin.dietermann@hive-brain.de";
-    };
-  };
-}

home/packages-base.nix

@@ -0,0 +1,10 @@
+{ pkgs, ... }:
+
+{
+  home.packages = with pkgs; [
+    neovim
+    nerd-fonts.jetbrains-mono
+    ripgrep
+    unzip
+  ];
+}

home/packages-marvin.nix

@@ -6,17 +6,13 @@
     claude-code
     gcc
     lua-language-server
-    neovim
-    nerd-fonts.jetbrains-mono
     nil
     nixpkgs-fmt
     nodejs
     obsidian
     pkg-config
-    ripgrep
     rustup
     steam
-    unzip
     xwallpaper
   ];
 }

home/zsh.nix

@@ -19,7 +19,7 @@
 
     shellAliases = {
       btw = "echo I use NixOS";
-      reload-nixos = "sudo nixos-rebuild switch --flake ~/nixos-dotfiles#hivebrain";
+      reload-nixos = "sudo nixos-rebuild switch --flake ~/nixos-dotfiles";
     };
   };
 }