feat(git): enable the module per host rather than by default

Defaulting the module on hid it from a host's config, leaving no single
place that lists what a host carries. Each host enables it explicitly
instead, at the cost of a step when adding one.

hosts/neogaia/default.nix

@@ -36,6 +36,7 @@
   modules.fish.enable = true;
   modules.fish.defaultShell = true;
 
+  modules.git.enable = true;
   modules.tmux.enable = true;
   modules.nvim.enable = true;
   modules.claude-code.enable = true;