pkg/logger: document INFO field-hiding as design contract, not limitation

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pkg/logger/handler.go

@@ -47,6 +47,10 @@ type writeState struct {
 //	INFO:  message: k=v, …    full fields — debug mode dumps everything
 //	WARN:  warning: msg: k=v, …
 //	ERROR: error:   msg: k=v, …
+//
+// INFO fields are intentionally hidden in normal mode: they are context for a
+// debug session, not alerts for the operator. Put fields on WARN or ERROR when
+// the reader needs them immediately to understand a problem.
 type humanHandler struct {
 	w      io.Writer
 	level  slog.Level