Fix plan: OverloadedStrings pragmas in test specs

docs/superpowers/plans/2026-08-20-concurrent-runtime.md

@@ -424,6 +424,7 @@ git commit -m "Add Runtime.Connection reader and writer actions"
 `test/RuntimeSpec.hs`:
 
 ```haskell
+{-# LANGUAGE OverloadedStrings #-}
 module RuntimeSpec (spec) where
 
 import Control.Concurrent (threadDelay)
@@ -577,6 +578,7 @@ git commit -m "Run controllers on their own threads over the bus"
 `test/SupervisorSpec.hs`:
 
 ```haskell
+{-# LANGUAGE OverloadedStrings #-}
 module SupervisorSpec (spec) where
 
 import Control.Concurrent (newEmptyMVar, putMVar, readMVar, threadDelay)