Workflows: Gate the Workflows UI behind an experimental feature flag

Register a 'workflows' experimental feature and gate the taskmaestro Workflows
UI behind it, following the registry pattern from #14081. The Workflows tree
collection is only added in RimProject::defineUiTreeOrdering when the feature is
enabled, and RimWorkflowCollection::rescanWorkflows() skips the eager discovery
scan (and its per-workflow Python introspection) when disabled.

ApplicationLibCode/Application/RiaExperimentalFeatures.cpp

@@ -48,6 +48,7 @@ const std::vector<RiaExperimentalFeatures::Feature>& RiaExperimentalFeatures::av
           "log level in preferences) to also see cvf's debug-level messages, e.g. OpenGL context and widget "
           "lifecycle events; note that debug level also logs one message per rendering pass and per part, every "
           "frame, so the log can grow quickly." },
+        { "workflows", "Workflows", "Discover and run taskmaestro workflows from the project tree." },
     };
 
     return features;