[All Platforms][CLI&UX] dcode -n "" returns exit code 0 instead of non-zero for empty prompt

## Description

Running `dcode -n ""` (empty prompt in non-interactive mode) prints the
correct error message "NemoClaw: empty non-interactive prompt for -n;
provide prompt text." but returns exit code 0 instead of non-zero.

Platform scope: Reproduced on ubuntu22, ubuntu26-gpu, dgx-station;
other platforms not tested but likely same behavior
(CLI code path is platform-independent).
Regression: Yes — worked in v0.0.73 (exit code 2), broken in v0.0.81
(exit code 0).

## Environment

```text
Device: CI runners (Ubuntu 22.04, Ubuntu 26.04, DGX Station)
OS: Ubuntu 22.04 / Ubuntu 26.04 / DGX Station
Architecture: x86_64
Node.js: v22.23.1
npm: 10.9.2
Docker: 29.x
OpenShell CLI: 0.0.72
NemoClaw: v0.0.81
OpenClaw: N/A (dcode agent)
```

## Steps to Reproduce

1. Install NemoClaw v0.0.81
2. Onboard a dcode sandbox: nemoclaw onboard --agent dcode --name test-dcode --non-interactive
3. Run: nemoclaw test-dcode exec -- dcode -n ""; echo "EXITCODE:$?"
4. Observe the exit code

## Expected Result

Exit code should be non-zero (exit 2 was observed in v0.0.73 day0 testing).
The error message is printed, so the error detection logic works — only
the exit code propagation is broken.

## Actual Result

```text
EXITCODE:0
NemoClaw: empty non-interactive prompt for -n; provide prompt text.

The error message is correct but exit code is 0.
```

## Logs

```text
AssertionError: expected 'EXITCODE:0NemoClaw: empty non-interac...'
not to contain 'EXITCODE:0'
Expected: "EXITCODE:0"
Received: "EXITCODE:0NemoClaw: empty non-interactive prompt for -n;
provide prompt text."
```