vars/password-store: replace passBackend option with passPackage

The `clan.core.vars.settings.passBackend` option has been replaced with
`clan.vars.password-store.passPackage` to provide better type safety and
clearer configuration.

Changes:
- Remove problematic mkRemovedOptionModule that caused circular dependency
- Add proper option definition with assertion-based migration
- Users setting the old option get clear migration instructions
- Normal evaluation continues to work for users not using the old option

Migration: Replace `clan.core.vars.settings.passBackend = "passage"`
with `clan.vars.password-store.passPackage = pkgs.passage`

nixosModules/clanCore/vars/default.nix

@@ -40,6 +40,18 @@ in
   };
 
   config = {
+    # Check for removed passBackend option usage
+    assertions = [
+      {
+        assertion = config.clan.core.vars.settings.passBackend == null;
+        message = ''
+          The option `clan.core.vars.settings.passBackend' has been removed.
+          Use clan.vars.password-store.passPackage instead.
+          Set it to pkgs.pass for GPG or pkgs.passage for age encryption.
+        '';
+      }
+    ];
+
     # check all that all non-secret files have no owner/group/mode set
     warnings = lib.foldl' (
       warnings: generator: