refactor: thread agent packages via option, drop overlay and forge-tools shim

nix/packages/default.nix

@@ -117,8 +117,8 @@ in
 {
   # All workspace binaries in one derivation — a symlinkJoin over the
   # per-bin packages, pure assembly with no extra compilation. The
-  # NixOS module's `pkgs.hyperhive` (= this) and `nix build .#` both
-  # land here.
+  # host module's `services.hyperhive.c0re.package` and `nix build .#`
+  # both land here.
   default = pkgs.symlinkJoin {
     name = "hyperhive";
     paths = lib.attrValues binPkgs;