feat(flake)!: convert everything to flake-parts-style flake modules

Use `agenix-rekey` flake module
Add `NixOS` module for `agenix-rekey`
- Default `masterIdentities`
- Default `localStorageDir`

nix/compat.nix

@@ -0,0 +1,10 @@
+let lock = builtins.fromJSON (builtins.readFile ../flake.lock);
+in
+import
+  (fetchTarball {
+    url = "https://github.com/edolstra/flake-compat/archive/${lock.nodes.flake-compat.locked.rev}.tar.gz";
+    sha256 = lock.nodes.flake-compat.locked.narHash;
+  })
+  {
+    src = ../.;
+  }