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

hosts/shared/nixos-pc.nix

@@ -1,4 +1,5 @@
 {
+  config,
   pkgs,
   lib,
   inputs,
@@ -151,7 +152,6 @@
   security.rtkit.enable = true;
 
   programs = {
-    ssh.startAgent = true;
     firefox.enable = true;
 
     gnupg.agent = {
@@ -168,6 +168,7 @@
   users.users.frank = {
     isNormalUser = true;
     description = "Frank LoTurco";
+    hashedPassword = config.sops.secrets.password_hash.path;
 
     extraGroups = [
       "networkmanager"