feat(flake): add packages and overlays

flake.nix

@@ -161,7 +161,7 @@
     with lib; {
       inherit lib;
 
-      pkgs = genAttrs systems pkgsFor;
+      pkgs = forEachSystem pkgsFor;
 
       # ╔══════════════════════════════════════════════════════════╗
       # ║                   NixOS Configurations                   ║
@@ -194,5 +194,22 @@
       checks = forEachSystem (system: {
         pre-commit-check = pre-commit-check system;
       });
+
+      packages = forEachSystem (system:
+        let
+          pkgs = pkgsFor system;
+        in
+        filterAttrs (_: isDerivation)
+          (overlay pkgs pkgs)
+      );
+
+      overlays =
+        let
+          overlayNames = attrNames (overlay null null);
+          mkOverlay = name: final: prev: (overlay final prev).${name};
+        in
+        (genAttrs overlayNames mkOverlay) // {
+          default = overlay;
+        };
     };
 }