changes to move opencode to nixpkgs unstable - more up to date and fixes breaking bugs

modules/system/core/default.nix

@@ -12,6 +12,7 @@
       user
       bootloader
       nix
+      overlays
       hardware
       locale
       nix-ld

modules/system/core/overlays.nix

@@ -0,0 +1,40 @@
+{
+  self,
+  inputs,
+  ...
+}: {
+  # Escape hatch for packages that move faster than the stable channel.
+  #
+  # `pkgs.unstable.<name>` resolves against nixos-unstable while the rest of the
+  # system stays on nixos-26.05. Keep the number of consumers small and prefer
+  # stable unless there is a concrete reason; every unstable package drags a bit
+  # of the unstable closure (glibc, stdenv) in alongside the stable one.
+  #
+  # Bump with `nix flake update nixpkgs-unstable`.
+  flake.overlays.unstable = _final: prev: {
+    unstable = import inputs.nixpkgs-unstable {
+      inherit (prev.stdenv.hostPlatform) system;
+      # Carries allowUnfree (set in nix-settings.nix) into the unstable set.
+      inherit (prev) config;
+    };
+  };
+
+  # This flake has two independent `pkgs` instances, and the overlay has to be
+  # registered on both or `pkgs.unstable` would exist in some modules only:
+  #
+  #   1. the NixOS module system's `pkgs` - plain modules such as
+  #      modules/features/bun, modules/system/core
+  #   2. flake-parts' perSystem `pkgs` - modules built with `moduleWithSystem`,
+  #      such as modules/attrs/development, which ignore `nixpkgs.overlays`
+  flake.nixosModules.overlays = {...}: {
+    nixpkgs.overlays = [self.overlays.unstable];
+  };
+
+  perSystem = {system, ...}: {
+    _module.args.pkgs = import inputs.nixpkgs {
+      inherit system;
+      overlays = [self.overlays.unstable];
+      config.allowUnfree = true;
+    };
+  };
+}