Finalize conversation and execution group documentation

exchange.go

@@ -4,7 +4,9 @@ import "go.maximumdirect.net/promptkit/internal/domain"
 
 const exchangeString = "promptkit.Exchange{opaque}"
 
-// Exchange is an immutable, in-process snapshot of a completed conversation.
+// Exchange is an immutable, reusable in-process snapshot of a completed
+// conversation. It is not a persistence or restart format. Different engines
+// can continue it, and independent branches do not alter one another.
 type Exchange struct{ internal *domain.Exchange }
 
 // ExchangeDetails contains a detached copy of an exchange's logical history.
@@ -14,7 +16,8 @@ type ExchangeDetails struct {
 	Messages    []RenderedMessage `json:"messages"`
 }
 
-// Details returns detached history, or zero details for an empty handle.
+// Details returns detached history, or zero details for a nil or empty handle.
+// Messages include caller and model content and require appropriate handling.
 func (e *Exchange) Details() ExchangeDetails {
 	if e == nil || e.internal == nil {
 		return ExchangeDetails{}