## Why
Fork-specific response items, including the subagent usage hint, are
appended directly to `InitialHistory::Forked`. This bypasses the normal
history insertion path that assigns missing response item IDs when
`Feature::ItemIds` is enabled, so the child could reconstruct and
persist those items without IDs.
## What changed
- When `Feature::ItemIds` is enabled, assign missing IDs to top-level
`ResponseItem`s while materializing `InitialHistory::Forked`, before
both reconstruction and persistence.
- Preserve existing IDs and use the same owned rollout items for live
history and persistence.
- Extract the existing single-item ID allocation logic for reuse by the
fork path.
- Add coverage that verifies a fork-only developer message receives the
same ID in live and persisted history with the feature enabled.
Normal history recording, compacted-history replacement, and fork
handling all continue to honor `Feature::ItemIds`. External-agent
imports, normal resume, and nested legacy compaction checkpoints are
unchanged.
## Testing
- `just test -p codex-core
record_initial_history_reconstructs_forked_transcript`
- `just test -p codex-core
record_initial_history_assigns_and_persists_id_for_forked_response_item`
pakrym-oai
806a4b66c9