fix(nix): remove mypy from pre-commit hooks

The mypy hook fails in Nix sandbox during flake check because uv
cannot discover Python installations in the isolated build environment.

Mypy is still run in CI via the unified ci.yml workflow, so type
checking is not lost. This change only affects the pre-commit hook.

Author: ryoppippi

flake.nix

@@ -51,14 +51,6 @@
               enable = true;
               package = config.treefmt.build.wrapper;
             };
-            mypy = {
-              enable = true;
-              name = "mypy";
-              entry = "${pkgs.uv}/bin/uv run mypy";
-              files = "^stackone_ai/";
-              language = "system";
-              types = [ "python" ];
-            };
           };
 
           devShells.default = pkgs.mkShell {