Add AI agent skill for Hardhat V2 to V3 migration (issue #8016)

[jose-compu]

• Because this PR includes a bug fix, relevant tests have been included.
• Because this PR includes a new feature, the change was previously discussed on an Issue or with someone from the team.
• I didn't do anything of this.

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AI Agent Skill for Hardhat V2 to V3 Migration

Closes #8016

Summary

Adds a SKILL.md file that AI agents (Cursor, Claude Code, Codex, etc.) can use to systematically migrate a Hardhat V2 project to V3.

The skill covers:

• Core migration: ESM, declarative config, configVariable(), explicit network connections
• Full plugin ecosystem: migration map for 16+ plugins (hardhat-deploy, hardhat-gas-reporter, hardhat-contract-sizer, hardhat-verify, @openzeppelin/hardhat-upgrades, solidity-coverage, TypeChain, etc.)
• Test migration: ESM syntax, hre.network.connect(), updated Chai matchers, network helpers, loadFixture pattern
• Custom tasks: builder pattern with .build(), hooks system replacing subtasks/extendConfig/extendEnvironment
• Troubleshooting: common errors and their fixes
• Quick reference table: V2 vs V3 equivalents at a glance

File placement note

I placed the skill at skills/hardhat-v2-to-v3-migration/SKILL.md but I'm not sure where skills should live in the Hardhat repo structure. Happy to move it wherever the team prefers.

Sources

Built from the official migration docs:

• https://hardhat.org/docs/migrate-from-hardhat2
• https://hardhat.org/docs/migrate-from-hardhat2/guides/mocha-tests
• https://hardhat.org/docs/reference/configuration
• https://hardhat.org/docs/learn-more/beta-status

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[Copilot]

Pull request overview

Adds an AI-agent-oriented migration “skill” document to guide upgrading Hardhat v2 projects to Hardhat v3, including core config/test changes and a plugin ecosystem mapping.

Changes:

• Introduces a new SKILL.md with a phased migration checklist (audit → core migration → plugins → tests → tasks → verification).
• Provides plugin migration map + examples for v3 declarative plugin/task configuration.
• Adds troubleshooting + quick-reference tables for common v2→v3 differences.

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[jose-compu]

@kanej thanks for you help, I am an external contributor. Does this SKILL.md goes here or somewhere else in another repo or web?