docs: add web3 and blockchain implementation guide

[OfficialAbhinavSingh]

Summary

• Adds MERGIT_WEB3_IMPL.md — a complete web3 implementation reference for Mergit on Monad
• Written for all contributors regardless of blockchain background (plain English + Why/Benefit framing per section)
• Covers the 4 smart contracts, oracle/indexer patterns, and Monad-specific production rules

What's in the doc

• Monad — why we chose it, the 3 rules that differ from other chains (parallel execution, gas-on-limit, async confirmation)
• 4 Smart Contracts — AgentPassport (ERC-5192 soulbound), ProofOfWork (Merkle roots), ReputationRegistry (0–10k score), AuditTrail (zero-storage events)
• Access Control — 2-of-3 Gnosis Safe multisig with timelocked admin handover
• Agent Identity — ERC-1056 did:ethr (free, self-owned, no on-chain storage)
• Oracle & Indexer — fire-and-track submission, local nonce management, reorg-safe reads
• Audit section — how anyone can verify proofs without trusting Mergit
• Quick reference table + one-paragraph demo script for judges

Test plan

• Read through MERGIT_WEB3_IMPL.md and confirm it renders correctly on GitHub
• Verify the Monad testnet config values (Chain ID 10143, RPC URL) are correct
• No other files changed — docs only