add dollar-cost averaging strategy

[CRF1408]

Adds a simple example demonstrating a DCA trading loop, showing how to use the client to select a market and repeatedly place small orders.

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[!NOTE] Introduces a minimal DCA strategy example demonstrating periodic order placement flow.

• New agents/examples/dca_strategy.py uses Polymarket client to fetch markets and pick the target via min(..., key=lambda m: m.spread)
• Configurable amount_per_purchase and interval_seconds; runs an infinite loop printing intended buys (placeholder for client.place_order(...))

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