Function migrate_legacy_abort_context 

pub fn migrate_legacy_abort_context(
    messages: &mut Vec<SharedMessage>,
    abort_context: &mut Option<String>,
) -> bool

Load-time migration for sessions saved before this change. A non-None abort_context means the saved history ends at the interrupted turn and the recap was waiting to be prepended to the next user message. The recap is DISCARDED (it is exactly the injection-shaped text this module replaces) and the marker is appended in its place.

Returns whether the session was migrated. Idempotent: abort_context is taken, so a second call is a no-op.