Add opencode package and install via base role

- Add packages/opencode with pre-built binaries from GitHub releases (v1.4.0)
- Support all 4 platforms (aarch64-darwin, x86_64-darwin, x86_64-linux, aarch64-linux)
- Add update-opencode flake app for automated version bumps
- Install opencode via home.roles.base so it's available on all machines
- Reformat flake.nix and packages with nixfmt

home/roles/base/default.nix

@@ -1,4 +1,9 @@
-{ config, lib, pkgs, ... }:
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
 
 with lib;
 
@@ -24,6 +29,7 @@ in
       tmux
       tree
       watch
+      custom.opencode
     ];
 
     # Automatic garbage collection for user profile (home-manager generations).