feat(configuration): update NixOS description and enhance direnv integration

home/emmatherock/agents.nix

@@ -6,7 +6,6 @@ in
 {
   home.packages = with agents; [
     antigravity-cli
-    codex
     claude-code
   ];
 }

home/emmatherock/core.nix

@@ -7,5 +7,12 @@
   programs.home-manager.enable = true;
   programs.btop.enable = true;
 
+  # Per-project toolchain resolution (nix-shell / flake devShell), used by
+  # nixvim's LSP servers so they don't need language toolchains installed globally.
+  programs.direnv = {
+    enable = true;
+    nix-direnv.enable = true;
+  };
+
   home.stateVersion = "25.11";
 }