Two review findings on the session-tree work:

- When `get_entries` failed, `lastKnownLeaf` was left pointing at an entry
  Pi may have already advanced past. The next successful capture then used
  that stale `since` cursor, so its window spanned several turns and the
  first user entry it found belonged to an earlier one, aiming rollback too
  far back. A failed capture now marks the cursor stale; the next capture
  re-syncs it and skips the turn-start ref for that one turn instead.
- Log only the length of pi's stderr. The chunk is unbounded remote output
  and can carry credentials or prompt text, matching the `stderrLength`
  form used elsewhere in the server.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>