Use WP Codebox v0.12.18 reviewer evidence

[chubes4]

Summary

• Advance all Docs Agent WP Codebox workflow, CI, fixture, assertion, and documentation references to v0.12.18.
• Document and verify the pre-sanitization canonical reviewer-evidence descriptor: one trusted artifact-relative transcript path with schema, digest, and size, revalidated before upload.
• Preserve all native package bytes.

Evidence

• Fixes compatibility with WP Codebox #1795.
• Regression evidence: hosted canonical reviewer-evidence run 29350690551.

How to test

1. Run php tests/validate-docs-agent-packages.php.
2. Run php tests/validate-external-native-package-sources.php.
3. Check out Automattic/wp-codebox at v0.12.18, set WP_CODEBOX_DIR to that checkout, then run php tests/validate-wp-codebox-run-agent-task-contract.php.
4. Set AGENTS_API_DIR to an Automattic/agents-api checkout, then run php tests/native-agent-import.php.
5. Run php tests/repair-docs-links-smoke.php.
6. Run actionlint.

AI assistance

• AI assistance: Yes
• Tool(s): OpenAI gpt-5.6-terra and OpenCode
• Used for: Updated the released workflow consumer references, contract assertions, and reviewer-evidence documentation; Chris remains responsible for review and merge.