Centralize publish transform policy validation

internal/publish/plan.go

@@ -102,13 +102,8 @@ func validateRequest(req Request) error {
 	if req.DestinationBackend == nil {
 		return fmt.Errorf("destination backend is required")
 	}
-	if !req.Publish.Source && !req.Publish.HTML {
-		return fmt.Errorf("publish source or html must be enabled")
-	}
-	if req.Publish.HTML {
-		if req.Transform.MarkdownToHTML == nil || !req.Transform.MarkdownToHTML.Enabled || req.Transform.MarkdownToHTML.Mode != config.TransformModeSidecar {
-			return fmt.Errorf("publish html requires markdown_to_html transform enabled with sidecar mode")
-		}
+	if err := config.ValidatePublishTransformPolicy(req.Publish, req.Transform); err != nil {
+		return fmt.Errorf("publish/transform policy: %w", err)
 	}
 	return nil
 }