feat(nix): migrate oatmeal

Using it as homebrew tap instead of Nix
(https://github.com/dustinblackman/oatmeal?tab=readme-ov-file#nix),
because I'm unable to install the NUR repository successfully 🙄

nix/home/default.nix

@@ -17,6 +17,7 @@ in
     ./lazydocker
     ./lazygit
     ./lf
+    ./oatmeal
     ./ripgrep
     ./ruby
     ./skhd