fix: run packaged CLI through Node

[chubes4]

Summary

• invoke the WordPress plugin's bundled CLI JavaScript entrypoint through Codebox's validated Node resolver
• avoid depending on executable mode surviving WordPress plugin ZIP extraction
• cover the installed-plugin shape with a deliberately non-executable bundled wrapper

Root cause

WP_Codebox_Agent_Sandbox_Runner::command_prefix() already resolved Node for the bundled wrapper, but discarded that result and executed the wrapper path directly. WordPress Studio extracted the packaged wrapper with mode 0644, causing every public agent task to exit 126 before sandbox creation.

Standalone CLI behavior and explicitly configured JavaScript paths are unchanged.

Verification

• php scripts/php-agent-runtime-execution-smoke.php
• npm run test:release-package-coverage
• PHP syntax checks
• git diff --check

AI assistance

OpenCode using openai/gpt-5.6-sol reproduced the public Codebox task failure, traced the command construction and package extraction boundary, implemented the repair and regression fixture, and ran verification. Chris Huber reviewed and owns the submitted changes.

Closes #2191.