Permit safe cross-run output declarations

internal/artifacts/remote_commit.go

@@ -114,7 +114,23 @@ func (m RemoteCommitManifest) ValidateForSessionPrefix(sessionPrefix string) err
 	if runPrefix == "" {
 		return fmt.Errorf("remote commit run prefix is required")
 	}
+	publishedDestinations := make(map[string]struct{})
 	for _, artifact := range m.Artifacts {
+		if artifact.Type == RemoteArtifactTypePublishedOutput {
+			identity, err := ParseImmutableRunObjectKey(sessionPrefix, artifact.DestinationKey)
+			if err != nil {
+				return fmt.Errorf("remote commit published output destination %q: %w", artifact.DestinationKey, err)
+			}
+			effectiveDestination := strings.TrimPrefix(identity.RelativePath, "published/")
+			if effectiveDestination == "" {
+				return fmt.Errorf("remote commit published output destination %q is empty", artifact.DestinationKey)
+			}
+			if _, duplicate := publishedDestinations[effectiveDestination]; duplicate {
+				return fmt.Errorf("remote commit declares duplicate published output destination %q", effectiveDestination)
+			}
+			publishedDestinations[effectiveDestination] = struct{}{}
+			continue
+		}
 		if !strings.HasPrefix(artifact.DestinationKey, runPrefix) {
 			return fmt.Errorf("remote commit artifact destination %q is outside immutable run prefix %q", artifact.DestinationKey, runPrefix)
 		}