Split configuration.nix and home.nix into per-domain modules

Break the two monolithic files into focused modules under system/
and home/ so each concern (boot, networking, xserver, picom, zsh,
git, rofi, etc.) lives in its own file. Clean up duplicates: drop
rofi from home.packages since programs.rofi.enable already installs
it, and move user-only apps (alacritty, obsidian, claude-code) from
environment.systemPackages to home.packages. Remove commented-out
boilerplate carried over from the install template.

home/zsh.nix

@@ -0,0 +1,25 @@
+{ ... }:
+
+{
+  programs.zsh = {
+    enable = true;
+    enableCompletion = true;
+    autosuggestion.enable = true;
+    syntaxHighlighting.enable = true;
+
+    history.size = 10000;
+    history.ignoreAllDups = true;
+    history.path = "$HOME/.zsh_history";
+
+    oh-my-zsh = {
+      enable = true;
+      plugins = [ "git" "sudo" ];
+      theme = "aussiegeek";
+    };
+
+    shellAliases = {
+      btw = "echo I use NixOS";
+      reload-nixos = "sudo nixos-rebuild switch --flake ~/nixos-dotfiles#hivebrain";
+    };
+  };
+}