feat: switch from flake-parts & devenv to rensa + add nixtest

nix/repo/apps.nix

@@ -0,0 +1,18 @@
+{
+  inputs,
+  cell,
+  ...
+}: let
+  inherit (inputs) pkgs nixlet-lib;
+  inherit (cell) nixlets;
+in {
+  upload = {
+    type = "app";
+    program =
+      (pkgs.callPackage nixlet-lib.uploadNixletsToGitlab {
+        projectId = "55602785";
+        nixlets = builtins.attrValues nixlets;
+      })
+      + "/bin/nixlets-upload";
+  };
+}