Add machines.keys config and reorganize key structure

- Add config.machines.keys for machine-specific keys (private keys live on that machine)
- Move desktop SSH key to machines.keys.desktop.ssh
- Fix extractName to preserve "yubikey" (only strip .key/.pub extensions)
- Rename key files for clarity (android -> graphone, primary -> yubikey)
- Add age yubikey key for encrypted backups
- Add README files to document key purposes
- Update all machine configs to import system config

src/system/machines/desktop/system.nix

@@ -13,7 +13,7 @@ in
       isNormalUser = true;
       extraGroups = config.user.groups
         ++ [ "video" "audio" "kvm" "libvirtd" "dialout" ];
-      openssh.authorizedKeys.keys = [ "${config.user.keys.ssh.android}" ];
+      openssh.authorizedKeys.keys = [ "${config.user.keys.ssh.graphone}" ];
     };
   };