Use WP Codebox v0.12.21 reviewer result projection

[chubes4]

Summary

Pin Docs Agent to the WP Codebox v0.12.21 reviewer-result projection and correct the evidence references.

What changed

• Keeps the reusable workflow tag pin at WP Codebox v0.12.21.
• Keeps the new reviewer-safe workflow-result projection capability flag, producer-contract assertions, and privacy documentation.
• Restores the fixture canonical successful reviewer-evidence run to 29350690551.
• Labels 29468956857 precisely: it executed WP Codebox v0.12.20, published Agents API #430, then failed during upload preparation. This hosted failure motivated WP Codebox #1814 and WP Codebox #1815; it is not a successful v0.12.21 reviewer-safe workflow-result run.

Compatibility

The reusable workflow tag changes to v0.12.21. Workflow inputs and outputs are unchanged. The uploaded reviewer result intentionally omits raw runtime/provider/model/tool data.

Evidence

• Source relationship: the v0.12.20 hosted failure above is motivating publication/failure evidence for the v0.12.21 projection behavior.
• Change kind: evidence semantics correction only; no additional runtime behavior.
• Verification capability: php tests/validate-docs-agent-packages.php and WP_CODEBOX_DIR=<v0.12.21 checkout> php tests/validate-wp-codebox-run-agent-task-contract.php passed.
• Runtime scope: the existing v0.12.21 pin, capability flag, producer-contract assertions, and privacy boundary remain unchanged.

AI assistance

• AI assistance: Yes
• Executor: OpenCode, model openai/gpt-5.6-terra
• Orchestrator: OpenAI gpt-5.6-sol
• Used for: Corrected evidence semantics, verified the pinned producer contract, and updated PR metadata; Chris reviews and owns the change.