fix(token-usage): fetch one flagged session with LIMIT 1

The incremental reconcile fetched the next flagged session by
materializing the whole grouped flag set and taking the head, which
rescanned the shrinking set once per session. Query one group directly
and start the throttle clock before the fetch so the lookup is charged
too.

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