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/config/keys/default.nix

@@ -0,0 +1,33 @@
+{ lib }:
+
+with builtins;
+let
+  extractName = filename:
+    let
+      # Remove .key extension
+      noKey = lib.removeSuffix ".key" filename;
+      # Remove .pub/.priv/.public/.private markers
+      noMarkers = replaceStrings
+        [ ".pub" ".priv" ".public" ".private" ]
+        [ "" "" "" "" ]
+        noKey;
+    in noMarkers;
+
+  constructKeys = dir: (
+    listToAttrs (
+      map (subdir: {
+        name = subdir;
+        value = listToAttrs (
+          map (file: {
+            name = extractName file;
+            value = readFile "${dir}/${subdir}/${file}";
+          }) (filter (file:
+            (readDir "${dir}/${subdir}").${file} == "regular" &&
+            lib.hasSuffix ".key" file
+          ) (attrNames (readDir "${dir}/${subdir}")))
+        );
+      }) (filter (node: (readDir dir).${node} == "directory") (attrNames (readDir dir)))
+    )
+  );
+in
+  constructKeys ./.