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`
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