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.

system/networking.nix

@@ -0,0 +1,6 @@
+{ ... }:
+
+{
+  networking.hostName = "hivebrain";
+  networking.networkmanager.enable = true;
+}