Update eligibility criteria link, add mainnet, wire per-network RPC

[cargopete]

Summary

• Wire per-network RPC endpoints so mainnet (Arbitrum One) and testnet (Arbitrum Sepolia) use separate RPCs
• Update eligibility criteria link to point to actual criteria documentation
• Fix dashboard URL and show full contract addresses in README

Changes

• Per-network RPC routing: RoundRobinRPC now takes a network parameter. Each environment gets its own RPC manager loaded from RPC_ENDPOINT_MAINNET / RPC_ENDPOINT_TESTNET, with fallback to generic RPC_ENDPOINT for backward compatibility.
• Eligibility Criteria link: Now points to https://github.com/graphprotocol/rewards-eligibility-oracle/blob/main/ELIGIBILITY_CRITERIA.md#eligibility-criteria (was previously linking to GIP-0079 as TBD)
• README: Full contract addresses (0x8ec2767a9d9ba02b4e09e8ff4fac2e14a340f304, 0x62c2305739cc75f19a3a6d52387ceb3690d99a99), fixed dashboard URL to hub.thegraph.foundation/reo/
• env.example: Network-specific RPC endpoints are now the primary configuration

Deployment note

The .env needs RPC_ENDPOINT_MAINNET (Arbitrum One RPC) and RPC_ENDPOINT_TESTNET (Arbitrum Sepolia RPC) configured for both environments to work.

Related

Depends on mainnet address being added to contracts repo addresses.json (separate PR).