bug: `sandbox download` exits 0 when the source path is rejected (outside the sandbox workspace)

Description

openshell sandbox download exits 0 when the download request is rejected because the source path is outside the sandbox workspace (/sandbox). The path-traversal guard correctly blocks the request and prints a user-facing error, but the failure is not propagated to the process exit code — scripts, test frameworks, and downstream tooling cannot detect the rejection.

Downstream report with full QA reproduction: NVIDIA/NemoClaw#7367. NemoClaw has added caller-side defenses (NVIDIA/NemoClaw#7371, NVIDIA/NemoClaw#7416 — probe the source and verify an artifact was written), but those cannot fix the CLI's own exit code, which is what that issue's repro exercises.

Reproduction Steps

1. Have a sandbox onboarded and running (single active gateway).
2. Run: openshell sandbox download {sandbox-name} /etc/passwd /tmp/p
3. Check the exit code: echo $?
4. Repeat with /sandbox/../etc/passwd and /var/lib/openshell/supervisor/creds.json — same behavior.

Observed:

$ openshell sandbox download {sandbox-name} /etc/passwd /tmp/p; echo $?
Downloading sandbox:/etc/passwd -> /tmp/p
Error:   x sandbox source path '/etc/passwd' is outside the sandbox workspace (/sandbox)
0

Expected: non-zero exit (e.g. 1) whenever the source path is rejected, so callers can detect the failure.

Environment

Device:        Ubuntu 24.04 VM (x86_64) and DGX Spark (aarch64)
OS:            Ubuntu 24.04 / Linux 6.8.0-124-generic
Architecture:  x86_64 / aarch64
Node.js:       v22.23.1
Docker:        Docker Engine 29.5.3
OpenShell CLI: openshell 0.0.85

Reproduced on both x86_64 and aarch64. Security-relevant: the rejected paths include credential files (/var/lib/openshell/supervisor/creds.json), and an exit-0 rejection means automated tooling treating exit codes as ground truth records a successful download that never happened.

Logs

See the observed output above; full QA logs in NVIDIA/NemoClaw#7367.

Start at the OpenShell CLI entry point for the sandbox download command and reproduce the rejected /etc/passwd, /sandbox/../etc/passwd, and credentials-path cases. Trace how the path-traversal error reaches the process boundary; done means each rejected source produces a non-zero exit while valid downloads retain their existing behavior.