feat: add delta mode for counter metrics using ClickHouse lag()

Counter metrics like chunks.exported.count are monotonically increasing.
Add mode=delta query parameter to the agent metrics API that computes
per-bucket deltas server-side using ClickHouse lag() window function:
max(value) per bucket, then greatest(0, current - previous) to get the
increase per period with counter-reset handling.

The chunks exported/dropped charts now show throughput per bucket
instead of the ever-increasing cumulative total.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/pages/AgentInstance/AgentInstance.tsx

@@ -75,7 +75,7 @@ export default function AgentInstance() {
   const { data: agentMetrics } = useAgentMetrics(
     agent?.instanceId || null,
     ['cameleer.chunks.exported.count', 'cameleer.chunks.dropped.count'],
-    60, timeFrom, timeTo,
+    60, timeFrom, timeTo, 'delta',
   );
 
   const feedEvents = useMemo<FeedEvent[]>(() => {