feat(FN-2979): add droid CLI probe module for node diagnostics

Added a new `droid-cli-probe` module to the dashboard package with test coverage, implementing a CLI probe capability for the droid system.

Fusion-Task-Id: FN-2979

packages/dashboard/src/__tests__/droid-cli-probe.test.ts

@@ -0,0 +1,43 @@
+import { describe, expect, it } from "vitest";
+import { probeDroidCli } from "../droid-cli-probe.js";
+
+/**
+ * These tests exercise the real probe — we deliberately do NOT mock
+ * `spawn`. The probe's job is to tell the truth about the local system,
+ * and mocking defeats that. Instead we:
+ *
+ *   1. Assert the shape is sound regardless of binary availability
+ *   2. Assert timeouts fire when the binary hangs (we don't have a
+ *      hanging binary fixture, so we cover the structural case only)
+ *   3. Let the suite pass whether or not `droid` is installed on the
+ *      test runner — both outcomes are legitimate.
+ *
+ * If you need mocked probe behavior for a higher-level route test, spy
+ * on `probeDroidCli` at the import boundary rather than shimming spawn.
+ */
+describe("probeDroidCli", () => {
+  it("returns a well-formed result whether or not droid is installed", async () => {
+    const result = await probeDroidCli();
+    expect(typeof result.available).toBe("boolean");
+    expect(typeof result.probeDurationMs).toBe("number");
+    if (result.available) {
+      // When available: version string should come back populated.
+      expect(typeof result.version).toBe("string");
+    } else {
+      // When unavailable: reason must be populated so the UI can render.
+      expect(typeof result.reason).toBe("string");
+    }
+  });
+
+  it("respects a short timeout", async () => {
+    // Not a true hang test (we don't have a hanging binary), but
+    // confirms the timeoutMs option wires through and probe completes
+    // in a bounded window when using a very small timeout.
+    const result = await probeDroidCli({ timeoutMs: 50 });
+    expect(result.probeDurationMs).toBeLessThan(5000);
+    // Either it completed fast enough or hit the timeout — both fine.
+    if (!result.available && result.reason?.includes("timed out")) {
+      expect(result.reason).toContain("50ms");
+    }
+  });
+});