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/packages-marvin.nix

@@ -0,0 +1,18 @@
+{ pkgs, ... }:
+
+{
+  home.packages = with pkgs; [
+    alacritty
+    claude-code
+    gcc
+    lua-language-server
+    nil
+    nixpkgs-fmt
+    nodejs
+    obsidian
+    pkg-config
+    rustup
+    steam
+    xwallpaper
+  ];
+}