fix(sdk): close MCP connections from Nanobot facade

The SDK opened MCP connections through AgentLoop.process_direct but
never called close_mcp, leaving stdio MCP generators to be finalized
during asyncio shutdown from a different task, producing a RuntimeError
about exiting a cancel scope in a different task.

Add aclose() that delegates to AgentLoop.close_mcp (which already
drains background tasks and closes MCP stacks), plus __aenter__ and
__aexit__ so the SDK works as an async context manager.

Fixes #4211

nanobot/nanobot.py

@@ -101,4 +101,13 @@ class Nanobot:
             messages=capture.messages,
         )
 
+    async def aclose(self) -> None:
+        """Release resources held by this instance (MCP connections, etc.)."""
+        await self._loop.close_mcp()
+
+    async def __aenter__(self) -> Nanobot:
+        return self
+
+    async def __aexit__(self, *exc: object) -> None:
+        await self.aclose()