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.

run_agent.py

@@ -132,7 +132,7 @@ from tools.browser_tool import cleanup_browser
 
 
 # Agent internals extracted to agent/ package for modularity
-from agent.memory_manager import sanitize_context
+from agent.memory_manager import flatten_message_content, sanitize_context
 from agent.error_classifier import FailoverReason
 from agent.redact import redact_sensitive_text
 from agent.model_metadata import (
@@ -2990,17 +2990,23 @@ class AIAgent:
             return
         if not (self._memory_manager and final_response and original_user_message):
             return
+        # Multimodal turns carry content as a list of typed parts; providers
+        # expect plain strings (see flatten_message_content).
+        user_text = flatten_message_content(original_user_message)
+        response_text = flatten_message_content(final_response)
+        if not (user_text and response_text):
+            return
         try:
             sync_kwargs = {"session_id": self.session_id or ""}
             if messages is not None:
                 sync_kwargs["messages"] = messages
             self._memory_manager.sync_all(
-                original_user_message,
-                final_response,
+                user_text,
+                response_text,
                 **sync_kwargs,
             )
             self._memory_manager.queue_prefetch_all(
-                original_user_message,
+                user_text,
                 session_id=self.session_id or "",
             )
         except Exception: