fix(compression): notify gateway users when summary generation fails

When auxiliary compression's summary LLM call fails (e.g. model 404,
auxiliary model misconfigured), the compressor still drops the selected
turns and inserts a static fallback placeholder — the dropped context
is unrecoverable.

Previously the only signal of this was a WARNING in agent.log. Gateway
users (Telegram/Discord/etc.) had no way to know context was lost
because the existing _emit_warning path requires a status_callback,
and the gateway hygiene path uses a temporary _hyg_agent with
quiet_mode=True and no callback wired up.

Changes:
- ContextCompressor: track _last_summary_fallback_used and
  _last_summary_dropped_count on each compress() call. Cleared at the
  start of compress() and on session reset.
- gateway/run.py hygiene: after auto-compress, inspect the temp
  agent's compressor; if fallback was used, send a visible ⚠️ warning
  to the user via the platform adapter (TG/Discord/etc.) including
  dropped count and the underlying error.
- gateway/run.py /compress: append the same warning to the manual
  compress reply so users running /compress see the failure too.

Acceptance:
- Summary success: no user-visible warning (unchanged).
- Summary failure on gateway hygiene: user receives a TG/Discord
  message with dropped count + error + remediation hint.
- Summary failure on /compress: warning appended to the command reply.
- CLI status_callback / _emit_warning path is untouched.
- Test coverage: two new tests verify the tracking fields are set on
  failure and cleared on subsequent success.

tests/agent/test_context_compressor.py

@@ -242,6 +242,72 @@ class TestSummaryFailureCooldown:
         assert mock_call.call_count == 1
 
 
+class TestSummaryFailureTrackingForGatewayWarning:
+    """When summary generation fails, the compressor must record dropped count
+    + fallback flag so gateway hygiene & /compress can surface a visible
+    warning instead of silently dropping context."""
+
+    def test_compress_records_fallback_and_dropped_count_on_summary_failure(self):
+        with patch("agent.context_compressor.get_model_context_length", return_value=100000):
+            c = ContextCompressor(model="test", quiet_mode=True, protect_first_n=2, protect_last_n=2)
+
+        msgs = [
+            {"role": "system", "content": "sys"},
+            {"role": "user", "content": "msg 1"},
+            {"role": "assistant", "content": "msg 2"},
+            {"role": "user", "content": "msg 3"},
+            {"role": "assistant", "content": "msg 4"},
+            {"role": "user", "content": "msg 5"},
+            {"role": "assistant", "content": "msg 6"},
+            {"role": "user", "content": "msg 7"},
+        ]
+
+        # Simulate summary LLM call failing — covers the 404 / model-not-found
+        # case from issue (auxiliary compression model misconfigured).
+        with patch("agent.context_compressor.call_llm", side_effect=Exception("404 model not found")):
+            result = c.compress(msgs)
+
+        assert c._last_summary_fallback_used is True
+        assert c._last_summary_dropped_count > 0
+        assert c._last_summary_error is not None
+        # Result must still be well-formed (fallback summary present).
+        assert any(
+            isinstance(m.get("content"), str) and "Summary generation was unavailable" in m["content"]
+            for m in result
+        )
+
+    def test_compress_clears_fallback_flag_on_subsequent_success(self):
+        mock_response = MagicMock()
+        mock_response.choices = [MagicMock()]
+        mock_response.choices[0].message.content = "summary text"
+
+        with patch("agent.context_compressor.get_model_context_length", return_value=100000):
+            c = ContextCompressor(model="test", quiet_mode=True, protect_first_n=2, protect_last_n=2)
+
+        msgs = [
+            {"role": "system", "content": "sys"},
+            {"role": "user", "content": "msg 1"},
+            {"role": "assistant", "content": "msg 2"},
+            {"role": "user", "content": "msg 3"},
+            {"role": "assistant", "content": "msg 4"},
+            {"role": "user", "content": "msg 5"},
+            {"role": "assistant", "content": "msg 6"},
+            {"role": "user", "content": "msg 7"},
+        ]
+
+        # First call fails, second succeeds — flag must reset on second compress.
+        with patch("agent.context_compressor.call_llm", side_effect=Exception("boom")):
+            c.compress(msgs)
+        assert c._last_summary_fallback_used is True
+
+        # Reset cooldown to allow retry on second compress
+        c._summary_failure_cooldown_until = 0.0
+        with patch("agent.context_compressor.call_llm", return_value=mock_response):
+            c.compress(msgs)
+        assert c._last_summary_fallback_used is False
+        assert c._last_summary_dropped_count == 0
+
+
 class TestSummaryPrefixNormalization:
     def test_legacy_prefix_is_replaced(self):
         summary = ContextCompressor._with_summary_prefix("[CONTEXT SUMMARY]: did work")