Forward base_ref to WP Codebox publication config

## 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:

```text
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`.

Start in .github/workflows/maintain-docs.yml, where the runner workspace JSON is built from base_ref. Find the existing contract coverage and extend it to verify that a non-main base_ref reaches both base and from. Confirm the generic WP Codebox fallback remains unchanged and that the workflow publishes against trunk.

Written by the indexing model from the issue text.