feat: upgrade memory flush system

- Use LLM to summarize discarded context into concise daily memory entries
- Batch trim to half when exceeding max_turns/max_tokens, reducing flush frequency
- Run summarization asynchronously in background thread, no blocking on replies
- Add daily scheduled flush (23:55) as fallback for low-activity days
- Sync trimmed messages back to agent to keep context state consistent

agent/memory/__init__.py

@@ -9,6 +9,7 @@ from agent.memory.manager import MemoryManager
 from agent.memory.config import MemoryConfig, get_default_memory_config, set_global_memory_config
 from agent.memory.embedding import create_embedding_provider
 from agent.memory.conversation_store import ConversationStore, get_conversation_store
+from agent.memory.summarizer import ensure_daily_memory_file
 
 __all__ = [
     'MemoryManager',
@@ -18,4 +19,5 @@ __all__ = [
     'create_embedding_provider',
     'ConversationStore',
     'get_conversation_store',
+    'ensure_daily_memory_file',
 ]