feat(cli): show a concrete example in the unknown sandbox action error

src/lib/cli/public-dispatch.ts

@@ -320,6 +320,7 @@ async function runPublicTranslationResult(
     case "unknownPublicAction":
       console.error(`  Unknown action: ${result.action}`);
       console.error(`  Valid actions: ${validSandboxActionsText()}`);
+      console.error(`  Example: ${CLI_NAME} ${opts.sandboxName ?? "<name>"} connect`);
       process.exit(1);
   }
 }

test/cli/unknown-sandbox-action.test.ts

@@ -0,0 +1,28 @@
+// SPDX-FileCopyrightText: Copyright (c) 2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+import fs from "node:fs";
+import os from "node:os";
+import path from "node:path";
+import { describe, expect, it } from "vitest";
+
+import { runWithEnv, testTimeoutOptions, writeSandboxRegistry } from "./helpers";
+
+describe("unknown sandbox action guidance (#755)", () => {
+  it("lists valid actions and a concrete example command", testTimeoutOptions(15_000), () => {
+    // The unknown-action path only triggers once the first token resolves to
+    // a registered sandbox; otherwise dispatch reports an unknown command.
+    const home = fs.mkdtempSync(path.join(os.tmpdir(), "nemoclaw-cli-unknown-action-"));
+    writeSandboxRegistry(home, "alpha");
+
+    // `2>&1` folds stderr into the captured output; the guidance is written to
+    // stderr before the command exits non-zero.
+    const r = runWithEnv("alpha definitely-not-an-action 2>&1", { HOME: home });
+
+    expect(r.code).not.toBe(0);
+    expect(r.out).toContain("Unknown action: definitely-not-an-action");
+    expect(r.out).toContain("Valid actions:");
+    // The concrete example uses the sandbox name the user actually typed.
+    expect(r.out).toContain("alpha connect");
+  });
+});