## Why
External agent session files can gain messages after their initial import. Re-importing those files should extend the existing Codex thread instead of creating a duplicate.
## What changed
- Map a changed source session back to its uniquely imported thread and append only the missing transcript suffix.
- Update the import ledger after verifying that the source and destination transcripts match.
- Defer the update when the target is active, archived, ambiguous, diverged, or otherwise unsafe to modify.
## Testing
Added unit and app-server coverage for suffix planning, ledger checkpointing, concurrent updates, and unsafe targets that must be deferred.
GitOrigin-RevId: 3d9e71cd66e8b31cf5128e8869063868bfb3eb05
stefanstokic-oai
bbbf396839