feat(common): support local git config overrides via includes

Add git include for ~/.gitconfig-local to allow per-machine overrides
(e.g. corporate user.name/email) without committing sensitive details.

Author: svnlto

common/git/default.nix

@@ -1,5 +1,5 @@
 # Unified cross-platform Git configuration
-{ pkgs, lib, ... }:
+{ config, pkgs, lib, ... }:
 
 let
   inherit (pkgs.stdenv) isLinux isDarwin;
@@ -59,6 +59,8 @@ in {
         "/Applications/1Password.app/Contents/MacOS/op-ssh-sign";
     };
 
+    includes = [{ path = "${config.home.homeDirectory}/.gitconfig-local"; }];
+
     signing = {
       key = signingKey;
       format = "ssh";