fix(agent): explain abnormal turn endings instead of blank/partial reply

When a turn ends abnormally after substantive tool calls (empty content
after retries, a partial/truncated stream, exhausted retries, or an
iteration/budget limit), the CLI/TUI response area was left blank or
showed only a fragment (e.g. "The") with no consolidated reason. The
internal turn_exit_reason values (empty_response_exhausted,
partial_stream_recovery, etc.) were never surfaced to the user.

Add a turn-completion explainer that mirrors the existing file-mutation
verifier footer: at turn end, map an abnormal turn_exit_reason to a
short, actionable message and either replace the bare "(empty)"
sentinel or append the reason after a partial fragment. Normal
text_response exits (e.g. a terse "Done.") stay quiet.

Gated by display.turn_completion_explainer (default on) with
HERMES_TURN_COMPLETION_EXPLAINER env override, matching the
file-mutation verifier seam.

Closes #34452

agent/conversation_loop.py

@@ -4480,6 +4480,55 @@ def run_conversation(
         except Exception as _ver_err:
             logger.debug("file-mutation verifier footer failed: %s", _ver_err)
 
+    # Turn-completion explainer.
+    # When a turn ends abnormally after substantive work — empty content
+    # after retries, a partial/truncated stream, a still-pending tool
+    # result, or an iteration/budget limit — the user otherwise gets a
+    # blank or fragmentary response box with no consolidated reason why
+    # the agent stopped (#34452).  Surface a single user-visible
+    # explanation derived from ``_turn_exit_reason``, mirroring the
+    # file-mutation verifier footer pattern above.
+    #
+    # Gate carefully so healthy turns stay quiet:
+    #   - ``text_response(...)`` exits never produce an explanation
+    #     (handled inside the formatter), so a terse ``Done.`` is silent.
+    #   - We only ACT when there is no genuinely usable reply this turn:
+    #     an empty response, the "(empty)" terminal sentinel, or a
+    #     suspiciously short partial fragment with no terminating
+    #     punctuation (e.g. "The").  A real short answer keeps its text.
+    if not interrupted:
+        try:
+            if agent._turn_completion_explainer_enabled():
+                _stripped = (final_response or "").strip()
+                _is_empty_terminal = _stripped == "" or _stripped == "(empty)"
+                # A short fragment that is not a normal text_response exit
+                # and lacks sentence-ending punctuation is treated as a
+                # truncated partial (the "The" case from #34452).
+                _is_partial_fragment = (
+                    not _is_empty_terminal
+                    and not str(_turn_exit_reason).startswith("text_response")
+                    and len(_stripped) <= 24
+                    and _stripped[-1:] not in {".", "!", "?", "。", "！", "？", "`", ")"}
+                )
+                if _is_empty_terminal or _is_partial_fragment:
+                    _explanation = agent._format_turn_completion_explanation(
+                        _turn_exit_reason
+                    )
+                    if _explanation:
+                        if _is_empty_terminal:
+                            # Replace the bare "(empty)"/blank sentinel with
+                            # the actionable explanation.
+                            final_response = _explanation
+                        else:
+                            # Keep the partial fragment, append the reason so
+                            # the user sees both what arrived and why it
+                            # stopped.
+                            final_response = (
+                                _stripped + "\n\n" + _explanation
+                            )
+        except Exception as _exp_err:
+            logger.debug("turn-completion explainer failed: %s", _exp_err)
+
     _response_transformed = False
 
     # Plugin hook: transform_llm_output