Restructure packages into per-program home/programs/ modules

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.

home/programs/prismlauncher.nix

@@ -0,0 +1,4 @@
+{ pkgs, ... }:
+{
+  home.packages = [ pkgs.prismlauncher ];
+}