Pin exact completion report prompt

[chubes4]

Summary

• pin Docs Agent f887e392 with the concrete valid-JSON completion report shape
• retain the accepted WP Codebox helper and package revisions

How to test

1. Check out Docs Agent f887e392ad93db11efb997ed3669171e67018403 and WP Codebox a6fe2d208e990a8d04104aa74aacbb8d1539fbc1.
2. Run DOCS_AGENT_DIR=/path/to/docs-agent WP_CODEBOX_DIR=/path/to/wp-codebox php tests/docs-agent-native-workflow-contract.php.
3. Run actionlint .github/workflows/*.yml.

Compatibility

This updates only immutable Docs Agent workflow provenance. Runtime APIs and source-delta semantics are unchanged.

Depends on Automattic/docs-agent#173

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode with GPT-5.6-sol
• Used for: Updated immutable producer provenance and assertions with Chris reviewing and owning the change.