Route Plugin Check through public WordPress workloads

## Problem

WP Codebox exposes `wordpress.plugin-check` in the runtime command catalog and implements it in the Playground runtime, but the canonical `wp-codebox/run-wordpress-workload` path does not route the command. A valid declarative workload returns `wp_codebox_wordpress_workload_step_unsupported` instead of running Plugin Check.

## Reproduction

Run a `wp-codebox/wordpress-workload-run/v1` request with a mounted plugin and this step:

```json
{
"command": "wordpress.plugin-check",
"args": ["plugin-slug=data-machine"]
}
```

Observed result: the safe WordPress workload runner marks the step skipped with `wp_codebox_wordpress_workload_step_unsupported`.

## Expected behavior

The public workload interface should route the catalogued `wordpress.plugin-check` recipe command through the disposable Playground implementation, install the official Plugin Check plugin, validate the mounted target, and return normalized `wp-codebox/plugin-check/v1` artifacts.

## Acceptance criteria

- `wp codebox run-wordpress-workload` accepts `wordpress.plugin-check`.
- The command runs only through the contained Playground runtime and preserves existing command policy enforcement.
- Raw and normalized Plugin Check artifacts are returned through the public workload result.
- Deterministic coverage proves a mounted plugin can be checked through the public CLI/ability path.

## AI assistance

- **Model:** GPT-5.6 Sol
- **Tool:** OpenCode
- **Used for:** Reproducing the public/runtime routing mismatch, inspecting the owning command implementations, and drafting this issue. Chris Huber remains responsible for the report and resulting change.

Start at the wp-codebox/run-wordpress-workload entry point and compare its command routing with the existing Playground implementation and runtime command catalog. Exercise a mounted plugin with the supplied wordpress.plugin-check request, then add deterministic CLI/ability coverage that verifies policy enforcement and that raw and normalized wp-codebox/plugin-check/v1 artifacts are returned.

Written by the indexing model from the issue text.