Advance native agent workflow to released observability stack

[chubes4]

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 Expose compact tool observability in chat metadata 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

• Expose compact tool observability in chat metadata agents-api#428 (merged)
• Project tool observability into reviewer transcripts wp-codebox#1809 (merged)
• Handle absent tool observability during upload wp-codebox#1811 (merged)
• WP Codebox v0.12.20 (released)