refactor: split common/nixos.nix and -/home.nix

Split these big files into modular files that
are easier to share and read on their own.

profiles/common/nixos/store.nix

@@ -0,0 +1,7 @@
+{ ... }:
+{
+  nix.optimise = {
+    automatic = true;
+    dates = [ "weekly" ];
+  };
+}