Forward base_ref to WP Codebox publication config

[chubes4]

Summary

Forward the reusable workflow’s explicit base_ref to runner workspace publication config as base, while retaining from: origin/<base_ref>.

Evidence

• Source relationship: .github/workflows/maintain-docs.yml already forwarded INPUT_BASE_REF to from; it omitted only the corresponding publication base.
• Change kind: minimal runtime configuration fix plus deterministic consumer contract coverage.
• Verification capability: the contract validator builds the exact runner-workspace JSON for baseRef=trunk and requires base: trunk and from: origin/trunk.
• Scope: no WP Codebox version, inputs, fallback behavior, or publication policy changed; the runtime addition is limited to the existing workspace-publication object.

Tests

1. Clone this branch and WP Codebox v0.12.27, then run WP_CODEBOX_DIR=/path/to/wp-codebox php tests/validate-wp-codebox-run-agent-task-contract.php.
2. Run php tests/validate-docs-agent-packages.php.
3. Run php tests/validate-external-native-package-sources.php.
4. Run php tests/repair-docs-links-smoke.php.
5. Run actionlint .github/workflows/*.yml.
6. Run git diff --check.

Compatibility Impact

Existing callers are compatible. Non-main base_ref values now reach the runner publication config explicitly; from remains origin-qualified. WP Codebox fallback behavior is unchanged.

Closes #156

AI assistance: OpenAI openai/gpt-5.6-terra.