Add agents overview documentation

[LennoxT]

Adds a short overview explaining what agents are and how they are used in the project.

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Note

Introduces high-level documentation for agents to help new contributors understand their role.

• Adds docs/AGENTS_OVERVIEW.md explaining what an “agent” is (modular decision/interaction component for Polymarket)
• Outlines core purposes: fetching market data, applying strategies to make decisions, and executing actions programmatically

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