Added automatic pruning for configured Postgres sinks

doc.go

@@ -78,7 +78,8 @@
 //     Sink abstractions + sink registry.
 //     Built-ins include stdout, NATS, and Postgres. For Postgres, downstream
 //     code registers table schemas/mappers while feedkit manages DDL, writes,
-//     and optional prune helpers.
+//     optional automatic retention pruning (via sink params.prune), and
+//     manual prune helpers. Postgres table schemas must declare PruneColumn.
 //
 // Typical wiring (daemon main.go)
 //