fixup: trim systemPackages to git only

nix flake check pulls rust tools hermetically. curl/jq in preStart
use absolute store paths. nix is part of any NixOS system by default.
git is the only package genuinely needed at runtime.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

nix/modules/hive-ci.nix

@@ -202,17 +202,11 @@ in
             "+${autoRegisterScript}"
           ];
 
-          # Tools available to workflow steps.
-          environment.systemPackages = [
-            pkgs.git
-            pkgs.nix
-            pkgs.cargo
-            pkgs.rustc
-            pkgs.rustfmt
-            pkgs.clippy
-            pkgs.curl
-            pkgs.jq
-          ];
+          # git is required by the runner's checkout step.
+          # Everything else (nix, rust tools) is either part of NixOS
+          # by default or pulled in hermetically by nix flake check.
+          # curl/jq in the autoregister preStart use absolute store paths.
+          environment.systemPackages = [ pkgs.git ];
         };
     };
   };