[Brev][Onboard] NVIDIA Cloud default model differs across OpenClaw, Hermes, and Deep Code harnesses

## Description

On the NemoClaw browser onboard page exported by a Brev launchable, selecting "NVIDIA Cloud" as the inference provider produces a different default model for each agent harness. Switching only the harness selection — with the provider left on NVIDIA Cloud — changes the NVIDIA model select to a different value each time, and for Hermes it falls back to the free-text "Other Model ID" entry instead of a named model.

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

## Environment

```text
Device: Brev launchable instance (n2d-standard-4, CPU only, no GPU)
OS: Ubuntu 22.04.5 LTS
Architecture: x86_64 (kernel 6.8.0-1064-gcp)
Node.js: v22.23.2
npm: 10.9.8
Docker: Docker version 29.1.3, build 29.1.3-0ubuntu3~22.04.2
OpenShell CLI: openshell 0.0.85
NemoClaw: nemoclaw v0.0.97
OpenClaw: N/A (defect is observed on the onboard page, before any
OpenClaw sandbox is created)
```

## Steps to Reproduce

1. Deploy the NemoClaw Brev launchable and open the `/onboard` page it exports.
2. On the first onboard screen, choose the middle option **"Set up in this browser"**.
3. Set the inference provider to **"NVIDIA Cloud"**.
4. Select agent harness **"OpenClaw"**; note the value in the NVIDIA model select.
5. Switch the agent harness to **"Hermes"**; note the value in the NVIDIA model select.
6. Switch the agent harness to **"Deep Code"**; note the value in the NVIDIA model select.

## Expected Result

Under the NVIDIA Cloud provider, all supported agent harnesses default to the same model, unless a per-harness default is explicitly specified by the PRD.

## Actual Result

Each harness defaults to a different model:

```text
OpenClaw -> "Nemotron 3 Super 120B"
Hermes -> "Other Model ID" free-text field, pre-filled with
nvidia/nemotron-3-nano-omni-30b-a3b-reasoning
Deep Code -> "Nemotron 3 Ultra 550B"
```

The Hermes default additionally does not resolve to any named entry in the NVIDIA model select; it lands on the "Other Model ID" fallback input.

The divergent default is not confined to the onboard UI — it is committed to the resulting sandbox. Completing the flow with the Deep Code harness on this instance produced a sandbox running model `nvidia/nemotron-3-ultra-550b-a55b` with provider `nvidia-prod`, matching the Ultra 550B default shown in the select.

## Logs

Sandbox state after completing onboard with the Deep Code harness, showing the committed default model:

```text
Sandboxes:
deepagents-code *
agent: langchain-deepagents-code model: nvidia/nemotron-3-ultra-550b-a55b
provider: nvidia-prod CPU sandbox policies: npm, pypi, huggingface, brew
```

Onboard-page model-select values were captured by direct UI observation; those steps produce no log output.