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.

agent/memory_manager.py

@@ -67,6 +67,45 @@ def sanitize_context(text: str) -> str:
     return text
 
 
+def flatten_message_content(content: Any) -> str:
+    """Flatten message content to plain text for memory providers.
+
+    Multimodal turns carry content as a list of ``{type: "text"|"image_url",
+    ...}`` parts; providers expect a string and feed it to regexes
+    (``sanitize_context``) and text APIs, so a list crashes the sync
+    (``expected string or bytes-like object, got 'list'``). Text parts are
+    joined, images become a ``[N image(s)]`` marker so the turn isn't
+    recorded as if the attachment never existed.
+    """
+    if content is None:
+        return ""
+    if isinstance(content, str):
+        return content
+    if isinstance(content, list):
+        text_bits: List[str] = []
+        image_count = 0
+        for part in content:
+            if isinstance(part, str):
+                if part:
+                    text_bits.append(part)
+                continue
+            if not isinstance(part, dict):
+                continue
+            ptype = str(part.get("type") or "").strip().lower()
+            if ptype in {"text", "input_text", "output_text"}:
+                text = part.get("text")
+                if isinstance(text, str) and text:
+                    text_bits.append(text)
+            elif ptype in {"image_url", "input_image"}:
+                image_count += 1
+        flattened = "\n".join(text_bits).strip()
+        if image_count:
+            note = f"[{image_count} image{'s' if image_count != 1 else ''}]"
+            flattened = f"{note} {flattened}" if flattened else note
+        return flattened
+    return str(content)
+
+
 class StreamingContextScrubber:
     """Stateful scrubber for streaming text that may contain split memory-context spans.