Codex builds fail on 1.38.0+ with ScopedFileSystemException on /r/*.md landing pages

## Summary
After **docs-builder 1.38.0** (released 2026-08-13 ~13:14 UTC), the production Codex assemble/deploy in `elastic/codex-environments` fails during `docs-builder codex build` with:

```
ScopedFileSystemException: Access denied: '.../.artifacts/codex/docs/r/.md'
... which is outside all configured scope roots.
```

This repeats for many generated repo landing pages (`agentic-interface-program.md`, `nightshift-program` loads then other repos fail, etc.).

## Impact
`codex.elastic.dev/r/*` has not successfully rebuilt since ~13:07 UTC (last good run used **1.37.0**). Link-index updates still fire `repository_dispatch`, but every subsequent build fails.

## Evidence
- Last success: https://github.com/elastic/codex-environments/actions/runs/31703316144 (docs-builder `1.37.0+8fcb754`)
- First failure: https://github.com/elastic/codex-environments/actions/runs/31706339136 (docs-builder `1.38.0+d216ca7`)
- Recent failure: https://github.com/elastic/codex-environments/actions/runs/31722230270

## Workaround
Pinning `docs-builder` to `1.37.0` in the Codex build workflow: https://github.com/elastic/codex-environments/pull/55

## Ask
Please either (1) merge the pin / restore builds, and (2) fix the ScopedFileSystem regression in docs-builder for generated Codex landing pages under `.artifacts/codex/docs/r/`.