Observability writers flushed every few seconds with MinBatchSize unset,
creating constant small parts and merge load. Enable MinBatchSize with
MaxFlushWait, batch access logs more aggressively, and shrink ClickHouse
background pools for 3c hosts.
Introduce internal/db/batchwriter as a reusable generic buffered writer
for per-domain ClickHouse flush pipelines, with unit tests and default
batch tuning aligned with audit log ingestion.
Add clickhouse-batchwriter agent skill and cross-references in AGENTS.md
and database-migration. Business layers are not wired yet.
ryan