fix: Add instructions about agent VS agent tool (no-changelog) (#18141)

packages/@n8n/ai-workflow-builder.ee/src/tools/prompts/main-agent.prompt.ts

@@ -147,6 +147,24 @@ Why: Vector Store needs three things: data (main input), document processing cap
 </rag_workflow_pattern>
 </node_connections_understanding>
 
+<agent_node_distinction>
+CRITICAL: Distinguish between two different agent node types:
+
+1. **AI Agent** (n8n-nodes-langchain.agent)
+   - Main workflow node that orchestrates AI tasks
+   - Accepts inputs: trigger data, memory, tools, language models
+   - Use for: Primary AI logic, chatbots, autonomous workflows
+   - Example: "Add an AI agent to analyze customer emails"
+
+2. **AI Agent Tool** (n8n-nodes-langchain.agentTool)
+   - Sub-node that acts as a tool for another AI Agent
+   - Provides agent-as-a-tool capability to parent agents
+   - Use for: Multi-agent systems where one agent calls another
+   - Example: "Add a research agent tool for the main agent to use"
+
+Default assumption: When users ask for "an agent" or "AI agent", they mean the main AI Agent node unless they explicitly mention "tool", "sub-agent", or "agent for another agent".
+</agent_node_distinction>
+
 <node_defaults_warning>
 ⚠️ CRITICAL: NEVER RELY ON DEFAULT PARAMETER VALUES ⚠️