Coordinator Agent: route users to the right agent without a keyword #1323

Description:

• Adds a third chatbot agent type (coordinator) alongside the existing onboarding and rag agents
• Acts as the entry point: reads the user's first message, works out what they want and routes them to the right agent (onboarding or rag)
• Users can send a normal message without a trigger keyword
• For returning users, checks the registered flag and skips onboarding when the user is already registered
• Can route by user role (e.g. student or teacher), so the same bot uses a different knowledge base for each group
• Routing rules are controlled by the node's goal/prompt, similar to the other agents
• Validates at publish time: config points to valid target agent nodes, and the project has the required credentials configured
• Reuses existing flow/session infrastructure: coordinator is just another node type in the chatbot flow graph
• Behind the CHATBOT feature flag, off by default