Pin opencode on WSL; Update

modules/overlays/opencode.nix

@@ -0,0 +1,13 @@
+# Overlay to provide a pinned version of opencode
+# This pins opencode to a specific nixpkgs commit to avoid issues
+# with the latest unstable version (e.g., segfaults).
+#
+# Usage: Pass nixpkgs-opencode input when applying overlay:
+#   overlays = [ (import ./modules/overlays/opencode.nix nixpkgs-opencode) ];
+
+nixpkgs-opencode: final: prev: {
+  opencode-pinned = (import nixpkgs-opencode {
+    inherit (prev) system;
+    config.allowUnfree = true;
+  }).opencode;
+}