Add prepare rerun guidance for unresolved previous-session analyze inputs

internal/stage/analyze.go

@@ -631,6 +631,15 @@ func resolveScriptoriumInput(
 	runtimeCatalog *artifacts.ArtifactCatalog,
 ) (string, bool, *artifacts.ResolvedSessionArtifact, error) {
 	source := strings.TrimSpace(inputCfg.Source)
+	if artifacts.IsPreviousSessionArtifactSource(source) {
+		if inputCfg.Required {
+			return "", false, nil, fmt.Errorf(
+				"required previous-session input source %q is unavailable; run narratio run-stage --force prepare",
+				source,
+			)
+		}
+		return "", false, nil, nil
+	}
 	switch source {
 	case "previous_session_artifact":
 		if strings.TrimSpace(inputCfg.Path) == "" {