fix(agent): register explainer config key + shorten footer prefix

Follow-up to the salvaged #34452 turn-completion explainer:
- Register display.turn_completion_explainer: True in DEFAULT_CONFIG so the
  setting is discoverable, matching the file_mutation_verifier precedent.
- Shorten the repeated footer prefix from 'Turn ended without a usable
  reply: ' to 'No reply: ' so the 10 reason variants don't all open with
  the same 8-word boilerplate.
- Update the 7 assertions that referenced the old prefix.

run_agent.py

@@ -2189,7 +2189,7 @@ class AIAgent:
         if reason.startswith("text_response"):
             return ""
 
-        prefix = "⚠️ Turn ended without a usable reply: "
+        prefix = "⚠️ No reply: "
         if reason == "empty_response_exhausted":
             return (
                 prefix