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>

cameleer3-server-core/src/main/java/com/cameleer3/server/core/storage/MetricsQueryStore.java

@@ -11,4 +11,11 @@ public interface MetricsQueryStore {
     Map<String, List<MetricTimeSeries.Bucket>> queryTimeSeries(
             String instanceId, List<String> metricNames,
             Instant from, Instant to, int buckets);
+
+    /** Counter mode: returns per-bucket deltas (max - previous max, floored at 0). */
+    default Map<String, List<MetricTimeSeries.Bucket>> queryTimeSeriesDelta(
+            String instanceId, List<String> metricNames,
+            Instant from, Instant to, int buckets) {
+        return queryTimeSeries(instanceId, metricNames, from, to, buckets);
+    }
 }