fix(compression): notify users when configured aux model fails even if main-model fallback recovers (#16775)

A misconfigured auxiliary.compression.model is a user-fixable problem that silent recovery would hide. The previous retry-on-main logic transparently swallowed aux-model failures whenever the fallback succeeded, leaving the user's broken config in place and racking up future failures.

Track the aux-model failure on the compressor alongside the existing fallback-placeholder fields:
- _last_aux_model_failure_model: str | None
- _last_aux_model_failure_error: str | None

Both are set at the moment the aux model errors (captured before summary_model is cleared for retry), regardless of whether the retry succeeds. Cleared at compress() start and on on_session_reset() so a clean run doesn't leak stale warnings.

Surface at three places:
- gateway hygiene auto-compress: ℹ note to the platform adapter (thread_id preserved)
- gateway /compress command: ℹ line appended to the reply
- CLI via _emit_warning: deduped on (model, error) so repeat compactions don't spam

Distinct from the existing ⚠️ dropped-turns warning — different severity, different emoji, explicit 'context is intact' reassurance.

gateway/run.py

@@ -4828,6 +4828,30 @@ class GatewayRunner:
                                                 "Failed to deliver compression-failure warning to user: %s",
                                                 _werr,
                                             )
+                                    # Separately: if the user's CONFIGURED aux
+                                    # model failed and we recovered by falling
+                                    # back to the main model, tell them — a
+                                    # misconfigured auxiliary.compression.model
+                                    # is something only they can fix, and
+                                    # silent recovery would hide it.
+                                    elif _comp is not None and getattr(_comp, "_last_aux_model_failure_model", None):
+                                        _aux_model = getattr(_comp, "_last_aux_model_failure_model", "")
+                                        _aux_err = getattr(_comp, "_last_aux_model_failure_error", None) or "unknown error"
+                                        _aux_msg = (
+                                            f"ℹ️ Configured compression model `{_aux_model}` "
+                                            f"failed ({_aux_err}). Recovered using your main "
+                                            "model — context is intact — but you may want to "
+                                            "check `auxiliary.compression.model` in config.yaml."
+                                        )
+                                        try:
+                                            _adapter = self.adapters.get(source.platform)
+                                            if _adapter and source.chat_id:
+                                                await _adapter.send(source.chat_id, _aux_msg, metadata=_hyg_meta)
+                                        except Exception as _werr:
+                                            logger.warning(
+                                                "Failed to deliver aux-model-fallback notice to user: %s",
+                                                _werr,
+                                            )
                                 finally:
                                     self._cleanup_agent_resources(_hyg_agent)
 
@@ -7377,6 +7401,11 @@ class GatewayRunner:
                 _summary_failed = bool(getattr(compressor, "_last_summary_fallback_used", False))
                 _dropped_count = int(getattr(compressor, "_last_summary_dropped_count", 0) or 0)
                 _summary_err = getattr(compressor, "_last_summary_error", None)
+                # Separately: did the user's CONFIGURED aux model fail
+                # and we recovered via main?  Surface that as an info
+                # note so they can fix their config.
+                _aux_fail_model = getattr(compressor, "_last_aux_model_failure_model", None)
+                _aux_fail_err = getattr(compressor, "_last_aux_model_failure_error", None)
             finally:
                 self._cleanup_agent_resources(tmp_agent)
             lines = [f"🗜️ {summary['headline']}"]
@@ -7392,6 +7421,13 @@ class GatewayRunner:
                     "with a placeholder; earlier context is no longer recoverable. "
                     "Consider checking your auxiliary.compression model configuration."
                 )
+            elif _aux_fail_model:
+                lines.append(
+                    f"ℹ️ Configured compression model `{_aux_fail_model}` failed "
+                    f"({_aux_fail_err or 'unknown error'}). Recovered using your main "
+                    "model — context is intact — but you may want to check "
+                    "`auxiliary.compression.model` in config.yaml."
+                )
             return "\n".join(lines)
         except Exception as e:
             logger.warning("Manual compress failed: %s", e)