Keep conversation history across user turns in all clients #275

Problem

Every client builds messages from scratch for each query, so a follow-up like "what about tomorrow?" has no context. Keeping the message list between turns takes a few lines per client, and it makes the demo the chatbot that every README says it is.

Tasks

• In all five clients, move messages out of process_query and onto the client struct or instance, and append to it. Keep the tool_use / tool_result pairs in the history so the model can refer back to them.
• Optional: add a clear command that resets the history, and mention it in the README next to quit.
• Mention in each README that history is kept for the whole session.

Acceptance criteria

• In all five clients, you can ask "get the forecast for 38.5816, -121.4944" and then "and the alerts for that state?", and the client works out the state from the previous turn.