loop: add not_done experiment

This attempts to get the LLM to introspect a bit more
while calling the done tool, encouraging it to change
itself mind if it thinks better of its progress
while working through the checklist.
diff --git a/experiment/experiment.go b/experiment/experiment.go
index 1e57f8d..b2e73cf 100644
--- a/experiment/experiment.go
+++ b/experiment/experiment.go
@@ -27,6 +27,10 @@
 			Name:        "all",
 			Description: "Enable all experiments",
 		},
+		{
+			Name:        "not_done",
+			Description: "Let the model backtrack halfway through a done tool call",
+		},
 	}
 	byName = map[string]*Experiment{}
 )