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/user/config/default.nix

@@ -14,7 +14,7 @@ in
         name = "bryan";
         email = "bryan@ramos.codes";
         shell = bash;
-        keys = import ./keys;
+        keys = import ./keys { inherit lib; };
 
         groups = [ "wheel" "networkmanager" "home-manager" "input" ];
         bookmarks = import ./bookmarks;