feat(home): enable ssh-agent user service

gnome-keyring 50 dropped its ssh component and gpg's ssh support is off,
so there was no agent at all — SSH_AUTH_SOCK unset, `ssh-add` failed.
Keys stay added-on-demand (`AddKeysToAgent = "no"` in the inventory).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

modules/home.nix

@@ -547,6 +547,12 @@ in
   # session has none otherwise; apps using libsecret/qtkeychain need it.
   services.gnome-keyring.enable = true;
 
+  # ssh-agent user service; sets SSH_AUTH_SOCK so `ssh-add` works in any shell.
+  # gnome-keyring above no longer ships an ssh component (dropped upstream), and
+  # gpg's ssh support is off, so this is the only agent. Keys are added on
+  # demand — nothing here presumes a particular ssh config.
+  services.ssh-agent.enable = true;
+
   services.mako = {
     enable = true;
     settings = {