Fix tomli-w package name in flake.nix

Changed tomli_w to tomli-w to match nixpkgs Python package naming convention (hyphens instead of underscores).

Author: claude

flake.nix

@@ -42,7 +42,7 @@
             jinja2
             pydantic
             pyyaml
-            tomli_w
+            tomli-w
           ];
 
           doCheck = false;