Add comprehensive GitHub Copilot instructions for eslint-plugin-primer-react with Node.js v20 support

[Copilot]

This PR adds a comprehensive .github/copilot-instructions.md file that provides GitHub Copilot with detailed instructions on how to work effectively in the eslint-plugin-primer-react codebase, updated to reflect the repository's Node.js v20 upgrade.

The instructions cover all essential aspects of working with this ESLint plugin repository:

Key Features

Setup and Bootstrap: Clear instructions for Node.js v20+ setup, dependency installation with npm ci, and confirms no build step is required since this is a direct JavaScript project.

Development Commands: All npm scripts with measured execution times:

• npm test - 5 seconds (286 tests)
• npm run lint - 1.5 seconds
• npm run format:check - 0.5 seconds
• npm run lint:md - under 1 second

Validation Workflows: Step-by-step instructions for validating changes before committing, including the complete sequence: lint → lint:md → format:check → test.

Manual Rule Testing: Working code examples showing how to test individual ESLint rules using Node.js and ESLint's RuleTester, with proper JSX parsing configuration.

Repository Navigation: Complete directory structure explanation covering src/rules/, src/rules/__tests__/, docs/rules/, src/configs/, and src/utils/.

Common Development Tasks: Detailed workflows for adding new rules and modifying existing ones, including the relationship between rule implementations, tests, and documentation.

Node.js v20 Alignment

All Node.js version references have been updated to reflect the repository's upgrade to v20:

• Setup instructions specify Node.js v20+ as the current standard
• Troubleshooting section updated to reference v20+
• Documentation aligns with the actual .nvmrc (v20) and CI workflows

Validation

All commands and patterns in the instructions have been thoroughly tested and validated:

• ✅ Dependency installation works as documented
• ✅ All test and lint commands execute with documented timing
• ✅ Manual rule testing patterns work correctly
• ✅ File structure matches documentation
• ✅ Complete validation workflow executes successfully
• ✅ Node.js v20 references validated against actual repository configuration

The instructions emphasize that this repository has very fast validation commands (all under 5 seconds except dependency installation), making it efficient for iterative development.

Fixes #377.

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[changeset-bot]

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[Copilot]

Pull Request Overview

This PR adds comprehensive GitHub Copilot instructions for the eslint-plugin-primer-react repository to help AI assistants work more effectively with the codebase. The instructions provide detailed guidance on setup, development workflows, testing patterns, and repository structure.

Key changes:

• Added complete development workflow documentation with command execution times
• Included manual rule testing patterns using Node.js and ESLint's RuleTester
• Documented repository structure and common development tasks

[Copilot]

The word 'tests' should not be bolded here as it's not a specific directory name. Consider using 'test files' or referencing the actual directory src/rules/__tests__/.

Suggested change

	3. Creating test files and using ESLint RuleTester in the **tests** files
	3. Creating test files and using ESLint RuleTester in the test files

[jonrohan]

@copilot I've just updated the repository to Node.js v20 can you update these docs?

[Copilot]

Updated the copilot-instructions.md file to reflect the Node.js v20 upgrade. All references to v18 have been updated to v20 throughout the documentation, including the setup instructions, troubleshooting section, and .nvmrc file reference. The changes have been validated with markdown linting.