[Ubuntu 24.04][Sandbox] destroy --cleanup-gateway leaves the shared gateway running and prints no gateway message on the last sandbox

## Description

On Linux, `nemoclaw {sandbox} destroy --cleanup-gateway --yes` against the last remaining sandbox exits 0 but leaves the shared OpenShell gateway service running and the gateway port bound. The destroy output contains no gateway-related messaging at all in this case — not a cleanup confirmation, and not the usual "Shared NemoClaw gateway preserved... pass `--cleanup-gateway`... next time" hint that normally prints when the flag is omitted.

Platform scope: Reproduced on Ubuntu 24.04 only; other platforms not tested.
Regression: Unknown — earlier versions not tested.

## Environment

```text
Device: Ubuntu 24.04 x86_64 server, NVIDIA GPU
OS: Ubuntu 24.04
Architecture: x86_64
Node.js: v22.22.3
npm: 10.9.8
Docker: 29.5.2
OpenShell CLI: 0.0.106
NemoClaw: v0.0.122
OpenClaw: 2026.7.1
```

## Steps to Reproduce

1. Onboard a fresh sandbox so exactly one sandbox is registered:
```bash
nemoclaw onboard --agent openclaw --name {sandbox} --non-interactive --fresh --yes
```
2. Confirm the gateway port is bound:
```bash
ss -tlnp | grep 8080
```
3. Run:
```bash
nemoclaw {sandbox} destroy --cleanup-gateway --yes
```
4. Confirm this really was the last sandbox, both via the registry and the live OpenShell state:
```bash
nemoclaw list
openshell sandbox list -g nemoclaw -o json
```
5. Re-check the gateway port and service state:
```bash
ss -tlnp | grep 8080
systemctl --user status nemoclaw-openshell-gateway.service
```

## Expected Result

Since this is confirmed as the last sandbox and `--cleanup-gateway` was passed explicitly, the shared gateway service should stop and the gateway port should be released. This is the behavior implied by NemoClaw's own destroy output on other runs (see Logs) — the hint text explicitly says passing `--cleanup-gateway` achieves cleanup.

## Actual Result

Destroy exits 0 with no gateway-related output at all. Step 4 confirms zero live/registered sandboxes (both `nemoclaw list` and `openshell sandbox list -g nemoclaw -o json` report empty). Despite that, step 5 shows the gateway service still active (running) and the gateway port still bound by the openshell-gateway process — the shared gateway was not cleaned up even though `--cleanup-gateway --yes` was passed on the confirmed last sandbox.

## Logs

Destroy output (no gateway-cleanup or gateway-preserved messaging present):
```text
Deleting sandbox '{sandbox}'...
Removed managed agent state volume for '{sandbox}'.
[services] cloudflared was not running
Sandbox '{sandbox}' destroyed
```

For comparison, a destroy WITHOUT `--cleanup-gateway` on the last sandbox normally prints:
```text
Shared NemoClaw gateway preserved. Re-run 'openshell gateway remove nemoclaw' to remove it,
or pass '--cleanup-gateway' / set NEMOCLAW_CLEANUP_GATEWAY=1 next time.
```

`systemctl --user status nemoclaw-openshell-gateway.service` after the `--cleanup-gateway` destroy:
```text
Active: active (running)
```

`ss -tlnp | grep 8080` after the `--cleanup-gateway` destroy:
```text
LISTEN 0 128 127.0.0.1:8080 0.0.0.0:* users:(("openshell-gatew",...))
LISTEN 0 128 {docker-bridge-ip}:8080 0.0.0.0:* users:(("openshell-gatew",...))
```