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/wsl/default.nix

@@ -3,6 +3,7 @@
 {
   imports = [
     ../../../user/config
+    ../../config
     ./system.nix
   ];
 }

src/system/machines/wsl/system.nix

@@ -9,8 +9,7 @@
       isNormalUser = true;
       extraGroups = config.user.groups;
       openssh.authorizedKeys.keys = [ 
-        "${config.user.keys.ssh.primary}" 
-        "${config.user.keys.ssh.windows}" 
+        "${config.user.keys.ssh.yubikey}" 
       ];
     };
   };