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/migrator/migrator.go

@@ -29,11 +29,17 @@ func dbType() string {
 	return "PostgreSQL"
 }
 
+const (
+	dialectSqlite   = "sqlite3"
+	dialectPostgres = "postgres"
+	cascadeSuffix   = " CASCADE"
+)
+
 func gooseDialect() string {
 	if !config.Config.Database.Enabled {
-		return "sqlite3"
+		return dialectSqlite
 	}
-	return "postgres"
+	return dialectPostgres
 }
 
 func migrationDir() string {