Replaces the monolithic packages-marvin.nix with one file per program
so hosts can opt in selectively. Adds docker, clamav, steam, and
audio keybindings for hivebrain. Fixes monitor order and switches
volume widget to PulseVolume.
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.
