yubikey-agent: switch to gui domain for hardware key PIN prompts

yubikey-agent triggers OS-level CCID/PIV PIN entry dialogs when SSH
authentication is requested. These system security prompts require
access to the macOS Aqua session (window server). Running in the user
domain (LimitLoadToSessionType=Background) blocks these prompts,
causing authentication to fail or hang.

Revert to the default gui domain so PIN prompts can render in the
graphical login session.

modules/services/yubikey-agent.nix

@@ -82,7 +82,6 @@ in
 
     launchd.agents.yubikey-agent = {
       enable = true;
-      domain = lib.mkDefault "user";
       config = {
         ProgramArguments = [
           "${cfg.package}/bin/yubikey-agent"

tests/modules/services/yubikey-agent/darwin/service.nix

@@ -23,8 +23,6 @@
       	</dict>
       	<key>Label</key>
       	<string>org.nix-community.home.yubikey-agent</string>
-      ${"\t"}<key>LimitLoadToSessionType</key>
-      ${"\t"}<string>Background</string>
       	<key>ProcessType</key>
       	<string>Background</string>
       	<key>ProgramArguments</key>