feat(git): carry the commit identity on every host

Default the module on rather than restating it per host, so a new host
commits without remembering the line. A host that should not carry a
personal identity sets enable to false.

hosts/neogaia/default.nix

@@ -36,7 +36,6 @@
   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;