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/server/system.nix

@@ -12,13 +12,12 @@
     backup = {
       enable = true;
       recipients = [
-        # TODO: Add your age recipients
-        # "${config.user.keys.age.yubikey}"
-        # "${config.user.keys.ssh.desktop}"
+       "${config.user.keys.age.yubikey}"
+       "${config.machines.keys.desktop.ssh}"
       ];
       destination = "gdrive:backups/server"; # TODO: configure rclone remote
       schedule = "daily";
-      keepLast = 7;
+      keepLast = 2;
     };
   };
 
@@ -27,7 +26,7 @@
       isNormalUser = true;
       extraGroups = config.user.groups;
       openssh.authorizedKeys.keys = [
-        "${config.user.keys.ssh.desktop}"
+        "${config.machines.keys.desktop.ssh}"
       ];
     };
   };