Implement Thread Message History

[murrayju]

When processing a message in a Slack thread, fetch the prior 10 messages and pass them to the Pydantic-AI agent as message_history. This provides the AI with conversation context without requiring MCP tool calls.

Changes:

• Add ThreadMessage type in types.py for representing thread messages
• Add fetch_thread_replies function in slack.py to fetch thread history
• Modify generate_response in agent.py to build and pass message_history
• User messages become ModelRequest, bot messages become ModelResponse

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[CLAassistant]

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