refactor(flake): centralize custom packages

src/flake/packages/default.nix

@@ -0,0 +1,8 @@
+{ pkgs, root }:
+{
+  pi-coding-agent = pkgs.callPackage ./pi-coding-agent.nix {
+    inherit root;
+  };
+
+  waybar = pkgs.callPackage ./waybar.nix { };
+}