chore(changeset): ADR-0087 disposition marker for the aggregation retirement

The two ledger entries were registered in the same commit; the gate
requires the answer in writing on the changeset itself (#6148).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011M7UwH25Unfi73UHim7ajY

Author: claude

.changeset/aggregation-array-string-agg-removed.md

@@ -78,3 +78,5 @@ ended — an aggregation that worked on one backend and failed on another is not
 capability — and both of those backends are inside the #5499 freeze. Their code
 is untouched; it is simply no longer reachable through a spec-valid request. On
 the dataset path nothing changes: `compileDataset` refused both by name already.
+
+<!-- adr-0087: registered query-array-string-agg-retired, dataset-measure-array-string-agg-removed -->