fix(memory): flatten multimodal content before provider sync

Multimodal turns carry message content as a list of typed parts
({type: "text"|"image_url", ...}). _sync_external_memory_for_turn
passed that list straight into MemoryManager.sync_all, and providers
feed it to regexes — Honcho's sync_turn calls sanitize_context, where
re.sub raised 'expected string or bytes-like object, got list'. Every
turn with an attached image silently never synced.

Flatten to plain text at the boundary: text parts joined, images noted
as an [N image(s)] marker so the attachment isn't erased from recall.
Fixing here covers all providers instead of patching each plugin.

tests/run_agent/test_memory_sync_interrupted.py

@@ -207,6 +207,57 @@ class TestSyncExternalMemoryForTurn:
         # sync_all still happened before the prefetch blew up.
         agent._memory_manager.sync_all.assert_called_once()
 
+    # --- Multimodal content flattening ----------------------------------
+
+    def test_multimodal_user_message_is_flattened(self):
+        """A turn with an attached image carries the user message as a
+        list of typed parts.  Providers feed the content to regexes
+        (sanitize_context), so a raw list raised ``expected string or
+        bytes-like object, got 'list'`` and the turn silently never
+        synced.  The boundary must flatten to text first."""
+        agent = _bare_agent()
+        agent._sync_external_memory_for_turn(
+            original_user_message=[
+                {"type": "text", "text": "what is in this screenshot?"},
+                {"type": "image_url", "image_url": {"url": "data:image/png;base64,abc"}},
+            ],
+            final_response="A terminal window showing a stack trace.",
+            interrupted=False,
+        )
+        agent._memory_manager.sync_all.assert_called_once_with(
+            "[1 image] what is in this screenshot?",
+            "A terminal window showing a stack trace.",
+            session_id="test_session_001",
+        )
+        agent._memory_manager.queue_prefetch_all.assert_called_once_with(
+            "[1 image] what is in this screenshot?",
+            session_id="test_session_001",
+        )
+
+    def test_multimodal_response_is_flattened(self):
+        agent = _bare_agent()
+        agent._sync_external_memory_for_turn(
+            original_user_message="describe it",
+            final_response=[{"type": "text", "text": "a cat"}],
+            interrupted=False,
+        )
+        agent._memory_manager.sync_all.assert_called_once_with(
+            "describe it", "a cat",
+            session_id="test_session_001",
+        )
+
+    def test_multimodal_with_no_text_at_all_skips(self):
+        """Unknown-typed parts flatten to an empty string — don't sync a
+        turn with no recoverable text."""
+        agent = _bare_agent()
+        agent._sync_external_memory_for_turn(
+            original_user_message=[{"type": "audio", "data": "..."}],
+            final_response="noted",
+            interrupted=False,
+        )
+        agent._memory_manager.sync_all.assert_not_called()
+        agent._memory_manager.queue_prefetch_all.assert_not_called()
+
     # --- The specific matrix the reporter asked about ------------------
 
     @pytest.mark.parametrize("interrupted,final,user,expect_sync", [