COUNT(DISTINCT col) on a large unfiltered table forces a full sort
and distinct pass. Often a perf surprise on prod. Warn when there's no
WHERE, LIMIT, or GROUP BY restricting the scope.

Should fail

SELECT COUNT(DISTINCT user_id) FROM events;

Should pass

SELECT COUNT(DISTINCT user_id) FROM events WHERE event_date >= CURRENT_DATE - 7;

Implementation hints

• Multi-line rule.
• Look for COUNT(DISTINCT ...) and require either WHERE or GROUP BY in
  the same statement.
• Severity: warning.

Estimated LOC: ~30 code + ~20 test.