feat: add n8n, beads, opencode packages

- Add pkgs/n8n/default.nix (v2.4.1)
- Add pkgs/beads/default.nix (v0.47.1, doCheck=false)
- Add pkgs/opencode/default.nix (v1.1.18)
- Include relax-bun-version-check.patch for opencode
- Register all packages in pkgs/default.nix

pkgs/beads/default.nix

@@ -0,0 +1,62 @@
+{
+  lib,
+  stdenv,
+  buildGoModule,
+  fetchFromGitHub,
+  gitMinimal,
+  installShellFiles,
+  versionCheckHook,
+  writableTmpDirAsHomeHook,
+}:
+buildGoModule (finalAttrs: {
+  pname = "beads";
+  version = "0.47.1";
+
+  src = fetchFromGitHub {
+    owner = "steveyegge";
+    repo = "beads";
+    tag = "v${finalAttrs.version}";
+    hash = "sha256-DwIR/r1TJnpVd/CT1E2OTkAjU7k9/KHbcVwg5zziFVg=";
+  };
+
+  vendorHash = "sha256-pY5m5ODRgqghyELRwwxOr+xlW41gtJWLXaW53GlLaFw=";
+
+  subPackages = ["cmd/bd"];
+
+  ldflags = ["-s" "-w"];
+
+  nativeBuildInputs = [installShellFiles];
+
+  nativeCheckInputs = [gitMinimal writableTmpDirAsHomeHook];
+
+  # Skip security tests on Darwin - they check for /etc/passwd which isn't available in sandbox
+  checkFlags =
+    lib.optionals stdenv.hostPlatform.isDarwin
+    ["-skip=TestCleanupMergeArtifacts_CommandInjectionPrevention"];
+
+  preCheck = ''
+    export PATH="$out/bin:$PATH"
+  '';
+
+  postInstall = lib.optionalString (stdenv.buildPlatform.canExecute stdenv.hostPlatform) ''
+    installShellCompletion --cmd bd \
+      --bash <($out/bin/bd completion bash) \
+      --fish <($out/bin/bd completion fish) \
+      --zsh <($out/bin/bd completion zsh)
+  '';
+
+  nativeInstallCheckInputs = [versionCheckHook writableTmpDirAsHomeHook];
+  versionCheckProgramArg = "version";
+  doInstallCheck = true;
+
+  # Tests require git worktree operations that fail in Nix sandbox
+  doCheck = false;
+
+  meta = {
+    description = "Lightweight memory system for AI coding agents with graph-based issue tracking";
+    homepage = "https://github.com/steveyegge/beads";
+    license = lib.licenses.mit;
+    maintainers = with lib.maintainers; [kedry];
+    mainProgram = "bd";
+  };
+})