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

packages/opencode/default.nix

@@ -0,0 +1,82 @@
+{
+  lib,
+  stdenv,
+  fetchzip,
+  patchelf,
+  glibc,
+}:
+
+let
+  version = "1.4.0";
+
+  srcs = {
+    aarch64-darwin = {
+      url = "https://github.com/anomalyco/opencode/releases/download/v${version}/opencode-darwin-arm64.zip";
+      sha256 = "0m97j2vln8yhhvnsjl92phx6dac24y7hgh75csmbkbhawkz9xm4l";
+    };
+    x86_64-darwin = {
+      url = "https://github.com/anomalyco/opencode/releases/download/v${version}/opencode-darwin-x64.zip";
+      sha256 = "17n04j06pdc2raxjm91y6p87gwpnra0liabpbjwdmyd1iqgqv0q8";
+    };
+    x86_64-linux = {
+      url = "https://github.com/anomalyco/opencode/releases/download/v${version}/opencode-linux-x64.tar.gz";
+      sha256 = "16117lwfj2lb8wjbq5cyf77vhi52ada5ys3212hjqw3qw3wrcc0r";
+    };
+    aarch64-linux = {
+      url = "https://github.com/anomalyco/opencode/releases/download/v${version}/opencode-linux-arm64.tar.gz";
+      sha256 = "06lvm1qiji74xdd3psqn6lwxak65gqsbmkib1pjb4n65f9246jwm";
+    };
+  };
+
+  src =
+    srcs.${stdenv.hostPlatform.system} or (throw "Unsupported system: ${stdenv.hostPlatform.system}");
+
+in
+stdenv.mkDerivation {
+  pname = "opencode";
+  inherit version;
+
+  src = fetchzip {
+    inherit (src) url sha256;
+  };
+
+  # Bun standalone binaries have JS code appended after the ELF sections
+  # stripping/patching would remove or corrupt this appended data
+  dontStrip = true;
+  dontPatchELF = true;
+
+  nativeBuildInputs = lib.optionals stdenv.isLinux [ patchelf ];
+
+  installPhase = ''
+    runHook preInstall
+
+    install -Dm755 $src/opencode $out/bin/opencode
+
+    runHook postInstall
+  '';
+
+  # Manually patch the interpreter for bun standalone binaries on Linux
+  postFixup = lib.optionalString stdenv.isLinux ''
+    interpreter="${glibc}/lib/${
+      if stdenv.hostPlatform.system == "aarch64-linux" then
+        "ld-linux-aarch64.so.1"
+      else
+        "ld-linux-x86-64.so.2"
+    }"
+    patchelf --set-interpreter "$interpreter" $out/bin/opencode
+  '';
+
+  meta = with lib; {
+    description = "Terminal-based AI coding assistant";
+    homepage = "https://opencode.ai";
+    license = licenses.mit;
+    maintainers = [ ];
+    platforms = [
+      "aarch64-darwin"
+      "x86_64-darwin"
+      "x86_64-linux"
+      "aarch64-linux"
+    ];
+    mainProgram = "opencode";
+  };
+}