Carry optional prompt labels through results

observations.go

@@ -100,11 +100,13 @@ type ObservedTokenUsage struct {
 // its output. Failure is classified metadata; RetriesExhausted is a separate
 // flag and does not replace the underlying failure kind.
 type ExecutionEvent struct {
-	Kind               ExecutionEventKind
-	RunID              string
-	Time               time.Time
-	PromptID           string
-	PromptVersion      string
+	Kind     ExecutionEventKind
+	RunID    string
+	Time     time.Time
+	PromptID string
+	// PromptVersion is the selected definition's descriptive label. Each event
+	// owns its pointer; nil means absent while a non-nil empty string is explicit.
+	PromptVersion      *string
 	ProfileID          string
 	BackendID          string
 	Model              string
@@ -131,7 +133,7 @@ func fromDomainExecutionEvent(value domain.ExecutionEvent) ExecutionEvent {
 	}
 	return ExecutionEvent{
 		Kind: ExecutionEventKind(value.Kind), RunID: value.RunID, Time: value.Time,
-		PromptID: value.PromptID, PromptVersion: value.PromptVersion, ProfileID: value.ProfileID,
+		PromptID: value.PromptID, PromptVersion: copyStringPtr(value.PromptVersion), ProfileID: value.ProfileID,
 		BackendID: value.BackendID, Model: value.Model, Generation: value.Generation,
 		Attempt: value.Attempt, Dispatched: value.Dispatched, CapacityWait: value.CapacityWait,
 		GenerationDuration: value.GenerationDuration, RetryDelay: value.RetryDelay, Duration: value.Duration,