Advance docs maintenance to WP Codebox v0.12.27

[chubes4]

Summary

• pin the developer-docs workflow to accepted Docs Agent merge 5302ef41
• pin CI producer validation to WP Codebox release commit 65cc5fb4
• validate the Docs Agent workflow uses released WP Codebox v0.12.27

Why

Acceptance run https://github.com/Automattic/build-with-wordpress/actions/runs/29642092608 passes install, build, verify, and drift but reports an opaque integrity mismatch. WP Codebox v0.12.27 and Docs Agent PR Automattic/docs-agent#155 preserve bounded changed-path evidence for the diagnostic replay.

How to test

1. Run pnpm install --frozen-lockfile.
2. Run pnpm build && pnpm verify.
3. Check out Docs Agent at 5302ef41556e7f681e1f1c95a9a3f37f3db4a9dc and WP Codebox at 65cc5fb4699cb7c2df13d04b4715c97097ac7565.
4. Run DOCS_AGENT_DIR=/path/to/docs-agent WP_CODEBOX_DIR=/path/to/wp-codebox pnpm test.

Compatibility

• No Build with WordPress public or extension API changes.
• This advances immutable workflow producers to the release with bounded integrity diagnostics.

AI assistance

• AI assistance: Yes
• Tool(s): OpenAI GPT-5.6 Sol via OpenCode
• Used for: Advanced immutable producer pins and contract constants, and ran verification with Chris reviewing and owning the change.