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/keys/age/README.md

@@ -0,0 +1,3 @@
+# Age Keys
+
+yubikey.pub.key - Cold storage backup for age encryption