Use copy-on-write WP Codebox runtime

[chubes4]

Summary

• Advance the reusable workflow and contract validation to WP Codebox v0.12.14.
• Pin the Agents API runtime closure and importer integration to merged Agents API #425 commit fbd5641d412af76a1b8288426a577e750838b4be.
• Assert imported package identity execution, sandbox-local copy-on-write workspace tools, host patch application/verification/publication, and provenance-only uploads.

Tracks WP Codebox #1784.

Verification

1. Run php tests/validate-docs-agent-bundle.php.
2. Run php tests/validate-external-native-package-sources.php.
3. Run php tests/repair-docs-links-smoke.php.
4. Run AGENTS_API_DIR=/path/to/agents-api php tests/native-agent-import.php with Agents API at fbd5641d412af76a1b8288426a577e750838b4be.
5. Run WP_CODEBOX_DIR=/path/to/wp-codebox php tests/validate-wp-codebox-run-agent-task-contract.php with WP Codebox v0.12.14.
6. Run actionlint .github/workflows/*.yml.
7. Run git diff --check.

Regression evidence: run 29311239264.

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.6-terra and OpenCode
• Used for: Updated workflow references, contract fixtures, validation assertions, and documentation; ran the verification suite.