gpg-agent: switch to gui domain for graphical pinentry support

gpg-agent is commonly configured with graphical pinentry programs
(pinentry-mac, pinentry-gnome3) on macOS that need access to the Aqua
session to display passphrase prompts. Running in the user domain
(LimitLoadToSessionType=Background) prevents these prompts from
rendering, causing passphrase entry to fail silently or hang.

Revert to the default gui domain so the agent runs in the graphical
login session where pinentry can interact with the window server.

modules/services/gpg-agent.nix

@@ -454,7 +454,6 @@ in
 
     launchd.agents.gpg-agent = {
       enable = true;
-      domain = lib.mkDefault "user";
       config = {
         ProgramArguments = [
           "${gpgPkg}/bin/gpg-agent"

tests/modules/services/gpg-agent/expected-agent.plist

@@ -16,8 +16,6 @@
 	</dict>
 	<key>Label</key>
 	<string>org.nix-community.home.gpg-agent</string>
-	<key>LimitLoadToSessionType</key>
-	<string>Background</string>
 	<key>ProcessType</key>
 	<string>Background</string>
 	<key>ProgramArguments</key>