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.

modules/git.nix

@@ -0,0 +1,25 @@
+{
+  config,
+  lib,
+  ...
+}:
+# git for the primary user, configured through home-manager.
+let
+  cfg = config.modules.git;
+  user = config.user.name;
+in
+{
+  options.modules.git.enable =
+    lib.mkEnableOption "git for the primary user, carrying the operator's commit identity";
+
+  config = lib.mkIf cfg.enable {
+    home-manager.users.${user}.programs.git = {
+      enable = true;
+
+      # Git will not guess a name and address from the login and hostname.
+      # Without these a commit fails outright with `Author identity unknown`.
+      settings.user.name = "alexion";
+      settings.user.email = "contact@alexion.dev";
+    };
+  };
+}