Use deterministic Docs Agent completion

[chubes4]

Summary

• pin Docs Agent 7b67d341, which generates completion artifacts from host-observed facts
• remove model response formatting from the effective acceptance path without changing the caller source delta

How to test

1. Check out Docs Agent 7b67d341eea16b75b8e055507d36a8d8e69a1fe3 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

The completion artifact is now host-generated and its v1 shape contains machine-observed facts only. Consumer inputs and documentation writable paths are unchanged.

Depends on Automattic/docs-agent#176

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.