docs: 3-way comparison — OpenCash vs AgentCash vs PayAI

[ConejoCapital]

Summary

• Expands COMPARISON.md from a 2-way (OpenCash vs AgentCash) to a 3-way comparison including PayAI Network
• PayAI occupies a different ecosystem layer (facilitator/settlement) vs OpenCash (merchant/proxy) and AgentCash (client/agent)
• Documents how the three projects complement each other — e.g. OpenCash can use PayAI as its facilitator for multi-chain gasless settlement
• Adds ecosystem layer diagram showing where each project fits

Key findings

• PayAI is an x402 facilitator (~14% market share), not an API proxy — it settles payments for merchants
• Supports 16 chains (Solana, Base, Polygon, Avalanche, Sei, etc.) — broadest chain coverage of any facilitator
• Gasless — PayAI covers all blockchain transaction fees
• Also runs an AI agent marketplace (ElizaOS + libp2p + OrbitDB)
• Risk: Anonymous team

Test plan

• Verify markdown renders correctly on GitHub
• All factual claims cross-referenced with PayAI docs and web sources

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