Advance Docs Agent publication base fix

[chubes4]

Summary

• pin developer-docs maintenance to accepted Docs Agent merge 53f43bc6
• keep WP Codebox pinned to released v0.12.27
• validate the immutable producer pair in CI

Why

The previous acceptance run reached publication but Docs Agent omitted the explicit trunk publication base, causing WP Codebox to fall back to main: https://github.com/Automattic/build-with-wordpress/actions/runs/29645661510

Docs Agent Automattic/docs-agent#159 now forwards base_ref to both workspace materialization and publication.

How to test

1. Run pnpm install --frozen-lockfile.
2. Check out Docs Agent at 53f43bc6433d2631750edabcb458a77db55aa8cb and WP Codebox v0.12.27 at its immutable release commit.
3. Run DOCS_AGENT_DIR=/path/to/docs-agent WP_CODEBOX_DIR=/path/to/wp-codebox pnpm test.
4. Run pnpm build && pnpm verify.

Compatibility

• No public API or extension behavior changes.
• The developer-docs workflow now publishes against its already configured trunk base instead of an implicit main fallback.

AI assistance

• AI assistance: Yes
• Tool(s): OpenAI GPT-5.6 Sol via OpenCode
• Used for: Advanced immutable producer pins, verified the producer contract and generated packages, and drafted this PR with Chris reviewing and owning the change.