feature: add post-quantum age (>=1.3.0) support

Add transparent support for post-quantum hybrid age keys (generated with age-keygen -pq). The agenix CLI now auto-detects age identity files in <del>~/.ssh/age.key and</del> ~/.config/age/*.key <del>, and the default identityPaths in both NixOS and home-manager modules now include ~/.ssh/age.key (and /etc/ssh/age.key for NixOS).</del> Changes: - pkgs/agenix.sh: Add detection of age identity files for decryption - <del>modules/age.nix: Add /etc/ssh/age.key to default identityPaths</del> - <del>modules/age-home.nix: Add ~/.ssh/age.key to default identityPaths</del> - Add post-quantum test keys and encrypted secret for integration testing - Update documentation to mention post-quantum key usage and how to specify them Post-quantum keys (AGE-SECRET-KEY-PQ-1) provide protection against future quantum attacks using ML-KEM-768 + X25519 hybrid encryption. This PR contains two commits: * the first one to bump nixpkgs to get age >=1.3.0 and adapting to changes in test execution .. there also is a separate PR in case that should go in independently: https://github.com/ryantm/agenix/pull/365 * the second one to enable the pq age support (which can be rebased in case #365 gets merged first)