Relax legacy ContractClock receipt constraint

db/migrations/002_reconcile_execution_receipts.sql

@@ -19,6 +19,8 @@ BEGIN
     ) THEN
         EXECUTE 'UPDATE execution_receipts
                  SET tool_name = COALESCE(tool_name, skill_name)';
+        EXECUTE 'ALTER TABLE execution_receipts
+                 ALTER COLUMN skill_name DROP NOT NULL';
     END IF;
 END
 $$;