Optimize agent context engineering to reduce cost

[yetibot-mps]

This PR optimizes the agent's context engineering to drastically lower costs, as requested:

• Reduces the thread context window to retrieve only the 20 most recent messages instead of paginating and fetching up to 500 messages, greatly reducing input tokens.
• Updates the agent prompt instructions to guide the agent to perform smart, low-cost SQL search queries or Discord history searches using the history command rather than consuming large thread histories.