## Why
Legacy rollouts can contain historical rollbacks, compaction checkpoints, and
subagent copies of parent history. Migrating those records verbatim can change
the visible conversation or the model context used when a thread resumes, and
can retain large amounts of duplicated subagent history.
## What changed
- Plan and replay historical rollbacks by logical turn, preserving surviving
lifecycle events and the correct compaction anchor for future resumes.
- Migrate subagent rollouts from a safe bounded replay when possible and record
the history boundary so copied parent context is not projected as child turns.
- Include archived and compressed rollouts while keeping their storage form and
location intact.
- Tolerate malformed, partial, oversized, and empty JSONL records during the
migration scan.
## Testing
Add coverage for rollback and compaction combinations, subagent replay bounds,
archived and compressed rollouts, recovery, concurrent maintenance, and damaged
JSONL input.
GitOrigin-RevId: 3948e16d43f75df8571b0a92af12a181d1d08fdf
Owen Lin
aac9f84247