ci: Advisor shadow synthesis transport failure leaves red candidate check

## Problem

The PR Review Advisor shadow-synthesis candidate repeatedly fails before analysis with an OpenShell transport error. The workflow still publishes a successful primary verdict, but the shadow candidate remains red and is easy to misread as a code-review blocker.

## Reproduction

- PR: #9932
- Exact head: `a2d79f7ff0bc7112292201ce75882fbfcd657b6a`
- Workflow run: https://github.com/NVIDIA/NemoClaw/actions/runs/32613820341
- Attempt 1 shadow job: https://github.com/NVIDIA/NemoClaw/actions/runs/32613820341/job/97131375398
- Attempt 2 shadow job: https://github.com/NVIDIA/NemoClaw/actions/runs/32613820341/job/97131793470

Both attempts end with:

```text
Error: × transport error
Command failed: openshell sandbox download pr-adv-synthesis .../artifacts/pr-review-advisor-shadow-synthesis .../artifacts/pr-review-advisor-shadow-synthesis
PR review advisor analysis did not complete: outcome=skipped
```

## Control evidence

On rerun, the primary Terra lane, Nemotron lane, all five specialist lanes, and Publish completed successfully. The published exact-head recommendation is `merge_as_is` with 0 blockers, 0 warnings, and 0 suggestions. This makes the remaining red candidate a shadow infrastructure failure rather than a finding against the PR.

This is distinct from #9980 incomplete required-session-read signature. That earlier failure was addressed by #9986 (`fce40c1`); the current attempts progressed past that point and failed during shadow artifact transport/download.

## Expected

The shadow candidate should either complete reliably or report infrastructure unavailability without presenting a red code-review candidate check that can be confused with a product regression.

Start with the shadow jobs in workflow run 32613820341 for PR #9932 at commit a2d79f7ff0bc7112292201ce75882fbfcd657b6a, focusing on the openshell sandbox download command and its transport error. Compare the shadow behavior with the successful primary and specialist lanes. Done means the shadow candidate completes reliably or reports infrastructure unavailability without appearing as a misleading red code-review result.

Written by the indexing model from the issue text.