Add a help command to guide users on available queries

Currently, new users may not know what kind of questions they can ask the chatbot. To improve usability, we should add a help command that provides a list of supported queries and topics.

Expected Behavior:

When the user types help, the chatbot should respond with a message like:

"You can ask me about workouts, meals, sleep, and motivation.
Try typing: 'workout plan', 'healthy meals', 'sleep tips', or 'motivate me'.