Preserve readonly mount targets for PHPUnit cwd resolution

## Problem

A readonly directory mount is staged to a private host path before Playground execution. The PHPunit command handler then receives its configured sandbox cwd unchanged, e.g. `/home/wpcom/public_html/bin/tests/isolated`, but that target is not readable in the sandbox after staging. The handler aborts before PHPUnit runs.

## Reproduction

1. Build WP Codebox at `6ad43b20c488166a08346b44e0e736579c1fd178`.
2. Run a `wordpress.phpunit` recipe with a readonly mount from a WPCOM runtime root to `/home/wpcom` and `cwd=/home/wpcom/public_html/bin/tests/isolated`.
3. Observe `RuntimeException: cwd is not a readable sandbox directory` with the staged host source reported under `/tmp/wp-codebox-inputs/...`.

## Expected

Readonly staging must preserve the declared sandbox mount target so commands can use declared `cwd`, test-root, and phpunit-xml paths.

## AI assistance

OpenAI GPT-5.6 Sol via OpenCode reproduced the isolated WP Codebox runtime failure, narrowed it to readonly mount staging versus PHPUnit cwd resolution, and drafted this tracker. Chris Huber remains responsible for the report.

Beginne damit, das wordpress.phpunit-Rezept bei Commit 6ad43b20c488166a08346b44e0e736579c1fd178 zu reproduzieren, und verfolge dann das Staging des readonly-Mounts bis zum PHPunit-Befehls-Handler. Überprüfe, dass das deklarierte sandbox-Mount-Ziel weiterhin für cwd-, test-root- und phpunit-xml-Pfade verwendet werden kann, und bestätige, dass PHPUnit ohne die Ausnahme wegen eines nicht lesbaren cwd ausgeführt wird.

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