docs: add verify-contracts reference covering Etherscan V2 API migration

[erhnysr]

Closes #35.

Problem

The Basescan V1 API (api.basescan.org/api) was permanently deactivated on August 15, 2025. Stale docs, LLM training data, and blog posts keep surfacing the old endpoint. Developers and AI agents hitting this get a cryptic error with no clear fix.

What this adds

A new reference file verify-contracts.md covering:

• V1 → V2 migration table with the exact error message and correct endpoint
• Foundry forge verify-contract (Mainnet/Sepolia, constructor args, linked libraries, --watch)
• foundry.toml V2 config replacing deprecated per-chain Basescan URLs
• Hardhat @nomicfoundation/hardhat-verify with apiKey as single string + customChains pointing to V2
• Sourcify as a decentralized alternative (no API key)
• Common errors table with causes and fixes
• Cross-reference to deploy-contracts.md

Verified

The V2 endpoint and Sourcify flows were tested on Base Sepolia before opening this PR (see #35 for reproduction details).

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1