refactor: centralize Nix-managed Codex assets and integrations

modules/capabilities/ai/_sources.nix

@@ -0,0 +1,10 @@
+{ pkgs }:
+{
+  ponytail = pkgs.fetchFromGitHub {
+    owner = "DietrichGebert";
+    repo = "ponytail";
+    rev = "356918eba965ee1eac64bd3a7f0dd02108350de5";
+    hash = "sha256-LPNMyHsri3+eeDmphEAKL1JgoRE4dLIPfZ4XZ+xu5UY=";
+  };
+  herdr = pkgs.llm-agents.herdr.src;
+}