test(version): bump CLI smoke-test timeout to 15s (flaky at 5s under load)

The gstack-next-version integration smoke test spawns a child process
that does git operations + sibling-worktree probing. Wall time hovers
4-5s on M-series Macs; flakes at exactly 5001-5002ms when the test
suite runs under load (bun's parallel scheduling). Bumping per-test
timeout to 15s eliminates the flake without changing test logic.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

test/gstack-next-version.test.ts

@@ -178,5 +178,5 @@ describe("integration (smoke)", () => {
     expect(Array.isArray(parsed.claimed)).toBe(true);
     expect(parsed).toHaveProperty("siblings");
     expect(parsed.siblings).toEqual([]); // --workspace-root null disabled scanning
-  });
+  }, 15_000); // Headroom over the 4-5s wall time of the spawned process under load
 });