feat(yubikey): configure GPG signing key and disable GCR ssh-agent

Set actual GPG key ID for git commit signing, disable GCR ssh-agent
socket so gpg-agent handles SSH auth, and trim redundant setup steps
from yubikey guide.

home.nix

@@ -1,4 +1,4 @@
-{ config, pkgs, ... }:
+{ config, pkgs, lib, ... }:
 
 {
   home.username = "l-wyatt";
@@ -121,6 +121,9 @@
     };
   };
 
+  # Disable GCR ssh-agent so gpg-agent handles SSH
+  systemd.user.sockets.gcr-ssh-agent.Install.WantedBy = lib.mkForce [];
+
   # GPG configuration
   programs.gpg = {
     enable = true;
@@ -141,7 +144,7 @@
   programs.git = {
     enable = true;
     signing = {
-      key = null; # Will be set after GPG key generation
+      key = "0xADCEAC560B498269";
       signByDefault = true;
     };
     settings = {