feat(git): declare the operator's commit identity (task 0015)

Git identity lived only in one checkout's local configuration on one
machine, so it was invisible to every other checkout and lost on a
reimage. Declare it as a module instead, enabled on neogaia.

It is a module rather than base plumbing so a host that should not carry
a personal commit identity can decline it.

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;