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.

hermes_cli/config.py

@@ -1202,6 +1202,13 @@ DEFAULT_CONFIG = {
         # class of over-claim that otherwise forces users to run
         # `git status` to verify edits landed.  Set false to suppress.
         "file_mutation_verifier": True,
+        # Turn-completion explainer.  When true (default), the agent appends a
+        # one-line explanation to its final response whenever a turn ends
+        # abnormally with no usable reply — empty content after retries, a
+        # partial/truncated stream, a still-pending tool result, or an
+        # iteration/budget limit.  Replaces the bare "(empty)" sentinel so the
+        # failure isn't silent from the UI's perspective.  Set false to suppress.
+        "turn_completion_explainer": True,
         "show_cost": False,       # Show $ cost in the status bar (off by default)
         "skin": "default",
         # UI language for static user-facing messages (approval prompts, a