fix: prevent duplicate memory consolidation tasks per session

Add a `_consolidating` set to track which sessions have an active
consolidation task. Skip creating a new task if one is already in
progress for the same session key, and clean up the flag when done.

This prevents the excessive API calls reported when messages exceed
the memory_window threshold — previously every single message after
the threshold triggered a new background consolidation.

Closes #751

nanobot/agent/loop.py

@@ -89,6 +89,7 @@ class AgentLoop:
         self._mcp_servers = mcp_servers or {}
         self._mcp_stack: AsyncExitStack | None = None
         self._mcp_connected = False
+        self._consolidating: set[str] = set()  # Session keys with consolidation in progress
         self._register_default_tools()
     
     def _register_default_tools(self) -> None:
@@ -318,8 +319,16 @@ class AgentLoop:
             return OutboundMessage(channel=msg.channel, chat_id=msg.chat_id,
                                   content="🐈 nanobot commands:\n/new — Start a new conversation\n/help — Show available commands")
         
-        if len(session.messages) > self.memory_window:
-            asyncio.create_task(self._consolidate_memory(session))
+        if len(session.messages) > self.memory_window and session.key not in self._consolidating:
+            self._consolidating.add(session.key)
+
+            async def _consolidate_and_unlock():
+                try:
+                    await self._consolidate_memory(session)
+                finally:
+                    self._consolidating.discard(session.key)
+
+            asyncio.create_task(_consolidate_and_unlock())
 
         self._set_tool_context(msg.channel, msg.chat_id)
         initial_messages = self.context.build_messages(