Advance docs maintenance to WP Codebox v0.12.26

[chubes4]

Summary

• pin the developer-docs workflow to accepted Docs Agent merge a7f820c2
• pin CI producer validation to WP Codebox release commit 2d5d000f
• validate the Docs Agent workflow uses released WP Codebox v0.12.26

Why

Acceptance run https://github.com/Automattic/build-with-wordpress/actions/runs/29627878516 passed install, build, verify, and drift, then exposed mutable .codebox state entering publication snapshots. WP Codebox v0.12.26 and Docs Agent PR Automattic/docs-agent#154 carry the owning-layer fix.

How to test

1. Run pnpm install --frozen-lockfile.
2. Run pnpm build && pnpm verify.
3. Check out Docs Agent at a7f820c231d11c79e598a458059cd34b9ad12dda and WP Codebox at 2d5d000f2f43670117b373d1b26a23181ee05480.
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 that excludes mutable .codebox runtime state from publication snapshots.

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.