Use WP Codebox v0.12.16 workspace runtime

[chubes4]

Summary

• advance the reusable workflow and producer-contract checkout to WP Codebox v0.12.16
• validate canonical task_input.workspaces handoff, external secret-filtered seed snapshots, actionable patch normalization, and preserved failed-result evidence
• retain the Agents API fbd5641d412af76a1b8288426a577e750838b4be pin and all Docs Agent package digests unchanged

Context

• Fixes Propagate runner workspace seeds into agent-task recipes wp-codebox#1786
• Fixes Snapshot runner workspace outside artifact topology wp-codebox#1788
• Released implementation: Fix agent task workspace change normalization wp-codebox#1790
• Regression evidence: workspace handoff, snapshot topology, v0.12.16 verification, and release

How to test

1. Run php tests/validate-docs-agent-packages.php.
2. Run php tests/validate-external-native-package-sources.php.
3. Check out Automattic/wp-codebox at v0.12.16, then run WP_CODEBOX_DIR=/path/to/wp-codebox php tests/validate-wp-codebox-run-agent-task-contract.php.
4. Check out Automattic/agents-api at fbd5641d412af76a1b8288426a577e750838b4be, then run AGENTS_API_DIR=/path/to/agents-api php tests/native-agent-import.php.
5. Run php tests/repair-docs-links-smoke.php, actionlint .github/workflows/*.yml, and git diff --check.

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.6-terra and OpenCode
• Used for: Updated the release pins, documentation, and producer-contract assertions; Chris directed the required runtime behavior and reviewed the resulting workflow scope.