fix: move MCP connected flag after successful connection to allow retry

The flag was set before the connection attempt, so if any MCP server
was temporarily unavailable, the flag stayed True and MCP tools were
permanently lost for the session.

Closes #889

nanobot/agent/loop.py

@@ -128,11 +128,20 @@ class AgentLoop:
         """Connect to configured MCP servers (one-time, lazy)."""
         if self._mcp_connected or not self._mcp_servers:
             return
-        self._mcp_connected = True
         from nanobot.agent.tools.mcp import connect_mcp_servers
-        self._mcp_stack = AsyncExitStack()
-        await self._mcp_stack.__aenter__()
-        await connect_mcp_servers(self._mcp_servers, self.tools, self._mcp_stack)
+        try:
+            self._mcp_stack = AsyncExitStack()
+            await self._mcp_stack.__aenter__()
+            await connect_mcp_servers(self._mcp_servers, self.tools, self._mcp_stack)
+            self._mcp_connected = True
+        except Exception as e:
+            logger.error("Failed to connect MCP servers (will retry next message): {}", e)
+            if self._mcp_stack:
+                try:
+                    await self._mcp_stack.aclose()
+                except Exception:
+                    pass
+                self._mcp_stack = None
 
     def _set_tool_context(self, channel: str, chat_id: str, message_id: str | None = None) -> None:
         """Update context for all tools that need routing info."""