fix(client): keychain -> ssh-agent

keychain is cool but is first of all unmaintained, and only really works
in the context of a shell. since it "dynamically" starts agents and
exports variables (with set -U to make matters worse), nothing exists
outside of the context of the shell which makes it impossible to sign
commits in gui apps (except vscode for some reason); using a classical
ssh-agent with the env var exported by hand simply works

programs/kde.nix

@@ -12,9 +12,10 @@
     };
   };
 
+  security.pam.services.sddm.kwallet.enable = true;
+
   environment = {
     systemPackages = with pkgs; [
-      flameshot
       gnome-calculator
       camasca.packages.${pkgs.system}.koi
     ];