feat(m4): add opencode to default packages

- Included `opencode` to the default package list for `aarch64-darwin`.
- Ensures availability of the `opencode` tool for users by default.

overlays/unstable/default.nix

@@ -2,6 +2,7 @@
 final: prev: {
   inherit (channels.unstable)
     gemini-cli
+    opencode
     # open-webui
     # vscode
     # nodejs_20

systems/aarch64-darwin/m4/default.nix

@@ -45,6 +45,7 @@ with lib.metacfg;
     aider-chat
     fabric-ai
     gemini-cli
+    opencode
   ];
 
   users.users.harald.shell = pkgs.fish;