Update native Docs Agent producer pins

[chubes4]

Summary

Port the conflicted #429 producer-pin update onto current main and consume Docs Agent #143 with WP Codebox v0.12.21.

What changed

• Pins the reusable Docs Agent workflow to merge commit 4c1043510ae71be4750cfde69777efc95ae0001e.
• Validates WP Codebox v0.12.21 at peeled release revision b5b1fc7eef4367d5e403de3e373df91d3722965b, including runtime-source and release-ref contracts.
• Supersedes the conflicted Run Docs Agent through the native executor #429 branch without disturbing current-main workflow changes.

How to test

1. Run git clone https://github.com/Automattic/docs-agent.git ../docs-agent-producer && git -C ../docs-agent-producer checkout 4c1043510ae71be4750cfde69777efc95ae0001e; expect The exact Docs Agent producer is checked out..
2. Run git clone https://github.com/Automattic/wp-codebox.git ../wp-codebox-producer && git -C ../wp-codebox-producer checkout v0.12.21; expect The exact released WP Codebox producer is checked out..
3. Run DOCS_AGENT_DIR=../docs-agent-producer WP_CODEBOX_DIR=../wp-codebox-producer php tests/docs-agent-native-workflow-contract.php; expect The native producer-chain contract passes..
4. Run php tests/no-product-imports-smoke.php; expect All product-neutrality assertions pass..

Compatibility

Workflow inputs and outputs remain compatible. The consumer advances immutable producer pins to Docs Agent #143 and WP Codebox v0.12.21; uploaded reviewer results now intentionally omit raw runtime/provider/model/tool payloads while retaining public publication and canonical transcript provenance.

Evidence

• CI expected: PHP tests
• CI expected: Workflow contracts
• Reviewer-resolvable source evidence: https://github.com/Automattic/agents-api/actions/runs/29468956857
• Reviewer-resolvable source evidence: Run Docs Agent through the native executor #429
• Reviewer-resolvable source evidence: Run Docs Agent #430
• Reviewer-resolvable source evidence: Use WP Codebox v0.12.21 reviewer result projection docs-agent#143
• Reviewer-resolvable source evidence: Project reviewer-safe workflow results wp-codebox#1815
• neutrality: Passed
• php-lint: Passed
• producer-chain: Passed

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode executor (openai/gpt-5.6-terra) orchestrated by OpenAI gpt-5.6-sol
• Model: openai/gpt-5.6-terra
• Used for: Ported the conflicted native Docs Agent consumer onto current main, updated immutable producer pins, and verified the producer contract; Chris reviews and owns the change.