[macOS][Onboard] one failed preflight sub-check reports every host capability as failed, including a working Docker daemon

## Description

`nemohermes onboard` on macOS reports every host-readiness capability (Docker availability, daemon reachability, runtime support, platform support, GPU checks) as failed simultaneously, even though Docker Desktop is confirmed running and reachable (`docker info`/`docker version` succeed). The onboard failure text also includes a "Headless Linux hosts often need explicit remote UI handling" suggestion, which does not apply to macOS — indicating the underlying preflight assessment short-circuited rather than evaluating each capability independently.

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

## Environment

```text
Device: MacBook (Apple Silicon)
OS: macOS 26.5.2 (Darwin 25.5.0, arm64)
Architecture: arm64
Node.js: v22.22.3
npm: 10.9.8
Docker: 29.5.2 (Docker Desktop 4.76.0)
OpenShell CLI: 0.0.106
NemoClaw: 0.0.116
OpenClaw: N/A (onboard not completed)
```

## Steps to Reproduce

1. On macOS with Docker Desktop installed and running (confirmed via `docker info` succeeding), run:
```bash
nemohermes onboard --name {sandbox} --non-interactive --fresh --yes --yes-i-accept-third-party-software
```
2. Observe the preflight step output.

## Expected Result

Preflight reports only the capabilities that are genuinely unavailable. A working, reachable Docker daemon should not be reported as unavailable, and macOS should not receive Linux-specific remediation text.

## Actual Result

```text
[1/8] Preflight checks
──────────────────────────────────────────────────
✗ System readiness could not confirm required capabilities: host.docker.available, host.docker.daemon_reachable, host.docker.runtime_supported, host.platform.supported, host.docker.storage_compatible, host.docker.storage_remediation_available, host.gpu.nvidia_available, host.gpu.container_toolkit_available, host.gpu.cdi_healthy.

Suggested fix:

- Review remote/headless UI settings (headless_remote_hint): Headless Linux hosts often need explicit remote UI handling if you want browser access.
Set `CHAT_UI_URL` when remote browser access matters.

Onboarding did not finish. Resume from the step that failed with:
nemohermes onboard --resume --name {sandbox}
Completed steps are skipped; pass --fresh instead to start over.
```

## Logs

Confirmed independently that Docker itself is healthy on this host:

```text
$ docker version
Server: Docker Desktop 4.76.0 (228118)
Engine:
Version: 29.5.2
API version: 1.54 (minimum version 1.40)
```