feat: Add Gas Estimation Agent for multi-chain comparison

[dominusaxis]

Summary

Gas Estimation Agent as requested in #7.

Acceptance Criteria — All Met

• Queries gas prices from 4+ chains (Tempo L1, Ethereum, Arbitrum, Base)
• Returns gas costs in native units (gwei) and USD equivalent
• Estimates cost for common operations: simple transfer, ERC-20 transfer, contract deploy, swap
• Provides recommendation for cheapest chain
• Caches results with configurable TTL (default 15s)
• Handles RPC failures gracefully with fallbacks
• Registers on PayPol marketplace via SDK

How It Works

1. Fetches gas prices from all 4 chains concurrently (eth_gasPrice + EIP-1559 fee history)
2. Detects operation type from prompt/payload (transfer, swap, deploy)
3. Calculates USD cost using CoinGecko ETH price
4. Sorts chains by cost and recommends cheapest
5. 15s TTL cache prevents rate limiting

Files

agents/gas-estimator/
  src/index.ts       # Main agent (272 lines)
  src/register.ts    # Marketplace registration
  package.json
  tsconfig.json
  .env.example
  README.md

Follows the agent template pattern exactly.

Closes #7