Advance native agent workflow to released observability stack

## Goal

Advance the reusable native Docs Agent workflow to the merged/released runtime stack and prove it can publish a real consumer documentation PR.

## Required updates

- Pin Agents API to merged commit `78e2dd409010f98fa4d26cdd72572117384ab18d` from Automattic/agents-api#428.
- Pin `Automattic/wp-codebox/.github/actions/run-agent-task` to `v0.12.20`.
- Set `wp_codebox_release_ref: v0.12.20` so action source and release artifact remain paired.
- Preserve the native WP Codebox path; do not restore the legacy Data Machine execution path.
- Run repository workflow contract/lint checks.
- Update consumer reusable-workflow SHA pins after merge.
- Trigger the hosted developer-docs workflow and require it to publish an actual documentation PR with sanitized tool observability.

## Dependencies

- Automattic/agents-api#428 (merged)
- Automattic/wp-codebox#1809 (merged)
- Automattic/wp-codebox#1811 (merged)
- WP Codebox v0.12.20 (released)

Start by locating the reusable native Docs Agent workflow and its workflow contract or lint checks. Update the Agents API commit, WP Codebox action and release pins while preserving the native path, then run the repository checks. Done means the hosted developer-docs workflow publishes an actual consumer documentation PR with sanitized tool observability and updated consumer SHA pins.

Written by the indexing model from the issue text.