PHPUnit workload fails when Composer platform_check references undefined STDERR

## Problem

A `wordpress.phpunit` workload fails before test discovery when a plugin's Composer autoloader includes the generated `vendor/composer/platform_check.php` in WP Codebox Playground.

## Reproduction

From an Extra Chill plugin worktree:

```sh
homeboy review test extrachill-api --path /var/lib/datamachine/workspace/extrachill-api@fix-121-route-affinity
```

WP Codebox 0.12.14 installs Composer autoloaders successfully, then the PHPUnit workload aborts:

```text
Recipe phase run_workloads failed: Recipe workflow steps[0] failed: wordpress.phpunit could not run: Error: Undefined constant \"STDERR\" at /wordpress/wp-content/plugins/extrachill-api/vendor/composer/platform_check.php:17
```

Run metadata:

- Homeboy run: `72c60725-ce63-4e50-9674-698de5c79457`
- WP Codebox runtime: `runtime-mrtlw45h-5gfo5q`
- WP Codebox packages: `0.12.14`
- Failure phase: `run_workloads`

## Expected

The PHPUnit workload should provide CLI standard-stream constants or load Composer autoloaders in a way that does not make a generated platform check crash before test discovery.

## Impact

Valid WordPress plugin tests cannot run in the sandbox whenever Composer generates a platform check that writes to `STDERR` on a failed requirement. The underlying platform-requirement message is also hidden by this secondary fatal error.

Start with the `wordpress.phpunit` workload entry point and reproduce the failure using the provided `homeboy review test` command. Trace when the Composer autoloader and generated `vendor/composer/platform_check.php` are loaded relative to CLI standard-stream setup. Done means PHPUnit reaches test discovery and the underlying platform-requirement message is not hidden by an undefined `STDERR` constant.

Written by the indexing model from the issue text.