feat: add comprehensive Copilot instructions for repository onboarding

[Copilot]

Adds .github/copilot-instructions.md to onboard the repository for GitHub Copilot coding agents. This comprehensive guide provides everything a coding agent needs to work efficiently with the impvol codebase without extensive exploration.

What's Included

Repository Overview: Complete description of impvol as an "Importable Volume" library that enables importing scripts and modules from memfs virtual filesystems using Node.js customization hooks.

Verified Build & Validation Instructions:

• Environment setup sequence: npm install → npm run build
• Testing with Node.js built-in test runner and tsx
• Complete linting pipeline (ESLint, knip, markdown, spelling, TypeScript)
• Formatting with Prettier
• Full validation sequence that matches CI requirements

Project Architecture & Layout:

• Source structure (6 TypeScript files in src/)
• Dual ESM/CJS build outputs via tshy
• Configuration files and their purposes
• Git hooks setup (Husky + lint-staged)

CI/CD Documentation: All 13 GitHub Actions workflows documented with their exact commands and purposes, including build/test matrix across Node.js versions and operating systems.

Dependencies & Architecture Details:

• Node.js 20.0.0+ requirement
• memfs peer dependency usage
• Worker thread architecture with known memory leaks (documented feature)
• Custom impvol:// protocol implementation

Common Issues & Workarounds: Documented solutions for typical build sequence issues, dual package considerations, and expected behaviors like memory leaks.

File Location Reference: Complete directory structure for easy navigation without exploration.

Validation

All commands documented in the instructions have been tested and verified to work correctly. The complete validation sequence passes all checks:

npm install && npm run build && npm run lint && npm run lint:knip && npm run lint:md && npm run lint:spelling && npm run tsc && npm test && npm run format -- --list-different

This file significantly reduces the time coding agents need to understand the repository structure and eliminates common build/test failures by providing precise, verified instructions.

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💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.