Forward base_ref to WP Codebox publication config

[chubes4]

Problem

Docs Agent accepts base_ref and uses it for recipe metadata and workspace materialization, but omits the corresponding base field from the runner_workspace JSON passed to WP Codebox. WP Codebox therefore applies its generic main fallback during publication.

This breaks consumers whose base branch is not main. Build with WordPress uses trunk; after all integrity and verification gates pass, publication fails with:

GitHub API GET /git/ref/heads/main failed with 404.

Acceptance evidence: https://github.com/Automattic/build-with-wordpress/actions/runs/29645661510

Root cause

.github/workflows/maintain-docs.yml builds runner workspace JSON with from: ("origin/" + $baseRef) but no base: $baseRef. WP Codebox publication reads config.base || config.base_branch || "main", so clone/materialization and publication disagree.

Acceptance criteria

1. Include base: $baseRef in the emitted runner workspace config.
2. Add deterministic contract coverage proving a non-main base_ref reaches both base and from.
3. Keep the generic WP Codebox fallback unchanged.
4. Build with WordPress manual PR-required maintenance publishes against trunk.