Option to archive (or make ephemeral) Codex threads created for delegate_task children

[studioetc]

Problem

Codex children started by delegate_task are created as ordinary, persisted Codex threads (originator: "T3 Code", source: "vscode") in the shared CODEX_HOME. They show up in the Codex app's recent list next to real conversations, and local tools that index ~/.codex/sessions can't tell them apart from main threads. Unlike Codex's own subagents, nothing in the rollout marks them as children.

Proposal

An opt-in setting for Codex delegate_task children, either:

• archive the native Codex thread (thread/archive) once the delegated task is terminal, or
• create it as an ephemeral thread.

Main (user-started) threads stay unchanged.

Notes

• Related: Archiving a thread should archive its native provider session #18975 (archive the native session when a T3 thread is archived). This one is about delegated children, which are never archived by the user.
• Workaround today: read nativeThreadRef.nativeId for thread:delegated-task:% rows in statev2.sqlite and archive or filter those IDs externally.