preview_session_migration

Inspects a source session without modifying it. The result includes resume-safety evidence, recoverable turn/checkpoint/task counts, active-session blocking, existing migration state, and the Narnia references that would carry forward.

Known incompatible histories include invalid or missing session.start records and event streams whose decoded content exceeds Copilot's current 536,870,888-character whole-file loader ceiling. The oversized case otherwise looks structurally valid, but direct --resume can silently create an unrelated blank session.

Parameter	Description
sessionId	Source Copilot session GUID

Use this before migrate_broken_session.