refactor: transition secrets to sops-nix instead of relying on hard-coded paths

hosts/shared/nixos-server.nix

@@ -1,4 +1,5 @@
 {
+  config,
   pkgs,
   ...
 }:
@@ -72,11 +73,10 @@
     };
   };
 
-  programs.ssh.startAgent = true;
-
   users.users.frank = {
     isNormalUser = true;
     description = "Frank LoTurco";
+    hashedPassword = config.sops.secrets.password_hash.path;
 
     extraGroups = [
       "wheel"