docs: add ERC-4626 vault example with Multi-Agent Orchestrator integration

[consumeobeydie]

Summary

This PR adds an example showing how to build an ERC-4626 tokenized vault for native USDC on Arc Testnet, including an agent-yield deposit path that integrates with the Multi-Agent Orchestrator example from this series.

What's included

• ArcUSDCVault.sol built on OpenZeppelin v5.6.1's ERC4626 base contract
• Owner-injected yield mechanism (depositYield) that increases share value for all holders
• Agent-tagged deposit function (depositForAgent) for orchestrator integration
• 13 passing tests covering deposit, withdraw, redeem, yield distribution, and access control

Live Verification

Deployed and tested with a real deposit on Arc Testnet:

• Contract: 0x6C13dA317B65474299F6fDee02daDd6626Eb2BFe
• Deposit tx: https://testnet.arcscan.app/tx/0x3db09d1f8cdc5f1506a48420c92bb6ee4fa6c90c31a6ab4441b41c92c2dc9c5d
• Result: 5 USDC deposited, 5,000,000 avUSDC shares minted

Full implementation

https://github.com/consumeobeydie/arc-vault

Related PRs

This is the 10th example added to this series.