feat: add dev shell with python3 and the benchmark skill (task 0004)

nix develop places benchmark-skill into ./.claude/skills/ (so /benchmark-skill
is available in this repo) and puts python3 on PATH to run the deterministic
core without a one-off nix shell.

flake.nix

@@ -92,6 +92,15 @@
               inherit pkgs;
             };
           };
+
+          # A working shell for developing skills in this repo.
+          # python3 runs the benchmark skill's deterministic core.
+          # The benchmark-skill is placed into ./.claude/skills/ on entry, so
+          # /benchmark-skill is available while working in this repo.
+          devShells.default = pkgs.mkShell {
+            packages = [ pkgs.python3 ];
+            shellHook = mkSkillsShellHook [ (skillPackages pkgs).benchmark-skill ];
+          };
         }
       );
 }