Updated sinks to add a functional postgres sink API.

doc.go

@@ -73,6 +73,9 @@
 //
 //   - sinks
 //     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.
 //
 // Typical wiring (daemon main.go)
 //