Codex starts encrypting prompts, uses ciphertext for inference instead

Environment

• Version: Upstream main after #26210 (Encrypt multi-agent v2 message payloads, merged 2026-06-05). This appears to affect versions that include that change and enable MultiAgentV2 (post-0.137.0).
• Subscription: Not subscription-specific.
• Model: Not model-specific. This concerns MultiAgentV2 spawn_agent, send_message, and followup_task message handling.
• Platform: Not platform-specific.
• Terminal emulator and version: Not terminal-specific.
• Codex doctor report: Not applicable. The regression is visible from the merged code behavior in #26210 rather than from local environment state.

Issue Description

#26210 makes MultiAgentV2 agent task/message payloads opaque to Codex by marking the model-facing message parameter as encrypted, storing only InterAgentCommunication.encrypted_content, and leaving InterAgentCommunication.content empty.

The encrypted delivery path is understandable as privacy hardening, but it also removes the human-readable task/message text from local rollout history, trace reduction, and parent-side audit/debug surfaces. That makes it difficult to answer basic questions such as:

• What task did this spawn_agent call give the child agent?
• What message was sent to a subagent?
• Why did a child thread exist when reviewing a rollout after the fact?

This is different from #26753, which reports request validation failures for encrypted tool schemas. This issue is about auditability and debuggability after the encrypted schema is accepted.

Steps to Reproduce

1. Have the model call spawn_agent, send_message, or followup_task.
2. Inspect the parent rollout/history/trace for the subagent task.
3. The task/message content is hidden behind ciphertext rather than being available as human-readable audit text.

Expected Behavior

Codex should preserve a human-readable, structured audit copy of the subagent task/message while still allowing encrypted delivery to the recipient model. A possible shape is to keep the encrypted message field for model delivery, but add a separate non-encrypted audit field for the readable task text. The audit field should be persisted in rollout/history/trace metadata so users and maintainers can inspect what was delegated without needing to decrypt model-delivery ciphertext.

Additional Information

The goal is not necessarily to revert encrypted delivery. The concern is that encrypted delivery should not fully remove local human auditability for subagent delegation.