Execute accepted WP Codebox helpers

[chubes4]

Summary

• pin the reusable workflow and helper implementation to WP Codebox a6fe2d20
• keep v0.12.29 as the packaged runtime asset release
• validate the independent helper and release refs against the published producer schema

How to test

1. Check out WP Codebox a6fe2d208e990a8d04104aa74aacbb8d1539fbc1.
2. Run php tests/validate-docs-agent-packages.php.
3. Run php tests/validate-external-native-package-sources.php.
4. Run php tests/validate-docs-agent-completion-contract.php.
5. Run WP_CODEBOX_DIR=/path/to/wp-codebox php tests/validate-wp-codebox-run-agent-task-contract.php.
6. Run actionlint .github/workflows/*.yml.

Compatibility

This adopts the additive wp_codebox_workflow_ref input. Packaged runtime assets remain v0.12.29 and consumer-facing Docs Agent inputs are unchanged.

Depends on Automattic/wp-codebox#1892

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode with GPT-5.6-sol
• Used for: Updated the reusable workflow integration, provenance, tests, and documentation with Chris reviewing and owning the change.