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.

tests/run_agent/test_turn_completion_explainer.py

@@ -159,7 +159,7 @@ def test_run_conversation_empty_exhausted_surfaces_explanation():
     # The user must NOT be left with a bare sentinel; the explanation wins.
     assert result["final_response"] != "(empty)"
     assert result["final_response"].strip() != ""
-    assert "without a usable reply" in result["final_response"]
+    assert "No reply:" in result["final_response"]
 
 
 def test_run_conversation_normal_reply_stays_quiet():
@@ -178,4 +178,4 @@ def test_run_conversation_normal_reply_stays_quiet():
 
     assert result["turn_exit_reason"].startswith("text_response")
     assert result["final_response"] == "Done."
-    assert "without a usable reply" not in result["final_response"]
+    assert "No reply:" not in result["final_response"]