feat(db): add ClickHouse batchwriter framework and skill

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.

internal/db/batchwriter/errs.go

@@ -0,0 +1,8 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package batchwriter
+
+import "errors"
+
+var errNilFlushFunc = errors.New("batchwriter: flush func is required")