fix: per-key mkDefault for hyperhive.packages so single-key overrides merge

flake.nix

@@ -72,11 +72,17 @@
           # Package wiring for agent containers — the harness modules
           # consume hyperhive's own packages via the `hyperhive.packages`
           # option (see nix/agent-modules/packages.nix); no overlay.
-          # `mkDefault` so a per-agent override of an individual key wins.
+          # The `mkDefault` is applied PER KEY (`mapAttrs`), not to the
+          # whole attrset: definition-level priority filtering runs
+          # before `attrsOf`'s per-key merge, so a whole-set `mkDefault`
+          # would be discarded entirely the moment an agent.nix
+          # overrides a single key. Per-key priorities make an
+          # individual override win while every other key keeps the
+          # flake default.
           agentPackages =
             { lib, pkgs, ... }:
             {
-              hyperhive.packages = lib.mkDefault {
+              hyperhive.packages = lib.mapAttrs (_: lib.mkDefault) {
                 inherit (self.packages.${pkgs.stdenv.hostPlatform.system})
                   hive-agent
                   hive-agent-mcp