Add first-class WordPress Plugin Check command

[chubes4]

Summary

• Add wordpress.plugin-check as a first-class WP Codebox command backed by the official WordPress Plugin Check plugin.
• Run Plugin Check through the official wp plugin check WP-CLI command instead of calling Plugin Check internals directly.
• Respawn WP Codebox runtime commands under Node's JSPI flags on Node 23+ so Playground avoids the Asyncify RuntimeError: unreachable crash triggered by PHPCS/proc_open().
• Normalize Plugin Check findings and capture raw/normalized Plugin Check artifacts.

Fixes #167.

Root cause

wp plugin check can install and register inside Playground, but the actual check crashes under Playground's Asyncify PHP-WASM build because Plugin Check invokes PHPCS and PHPCS uses proc_open(). WP Codebox imports @wp-playground/cli programmatically, which bypassed the Playground CLI entrypoint's JSPI respawn behavior.

Relevant upstream trackers:

• CLI: Auto-enable JSPI flags on Node.js 23+ WordPress/wordpress-playground#3407
• Asyncify is deprecated. Let's remove Asyncify builds once JSPI support is the baseline WordPress/wordpress-playground#1872

Verification

• npm run build
• npm run discovery-command-smoke
• npm run plugin-check-normalization-smoke
• npm run check
• Manual targeted run: npm run wp-codebox -- run --mount ./examples/simple-plugin:/wordpress/wp-content/plugins/simple-plugin --command wordpress.plugin-check --arg plugin-slug=simple-plugin --arg checks=plugin_header_fields --artifacts ./artifacts/plugin-check-command-probe --json
• Manual full run: npm run wp-codebox -- run --mount ./examples/simple-plugin:/wordpress/wp-content/plugins/simple-plugin --command wordpress.plugin-check --arg plugin-slug=simple-plugin --artifacts ./artifacts/plugin-check-full-command-probe --json

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Investigated the Playground/Plugin Check crash, implemented the JSPI respawn and official WP-CLI Plugin Check path, added smoke coverage, and ran verification. Chris remains responsible for review and merge.