docs: add comprehensive .github/copilot-instructions.md with build, test, and validation guidance

[Copilot]

This PR adds comprehensive instructions for GitHub Copilot coding agents to work efficiently with the impvol repository. The instructions are designed to minimize exploration time, reduce build failures, and improve the quality of agent-generated pull requests.

What's Added

A detailed .github/copilot-instructions.md file that provides:

Repository Overview

• Clear description of impvol's purpose as an importable volume for virtual filesystems
• Technology stack details: TypeScript/Node.js v20+, dual ESM/CJS package built with tshy
• Key dependencies and memory considerations (worker thread limitations)

Build & Validation Workflow

• Complete command sequences with timing estimates for all operations
• Critical setup requirements including the --foreground-scripts flag for npm ci
• Comprehensive validation commands: build, test, lint, type-check, markdown, spelling, unused code detection
• Build artifact management and clean build procedures

Project Architecture

• Source code layout with detailed descriptions of key files and their purposes
• Configuration files documentation for TypeScript, ESLint, Prettier, and other tools
• CI/CD workflows covering all 13 GitHub Actions workflows and their purposes
• Git hooks and code quality setup with husky, lint-staged, and commitlint

Agent Guidelines

• Validated command sequences that have been tested end-to-end
• Common failure patterns and how to avoid them (e.g., running tsc directly vs npm run tsc)
• Trust-first approach to minimize unnecessary exploration

Validation

All instructions have been thoroughly tested by:

• Running complete clean environment setup from scratch
• Executing all documented command sequences successfully
• Verifying all linting rules pass (markdown, spelling, formatting)
• Testing failure scenarios to validate warnings and troubleshooting guidance

The instructions follow the repository's existing standards and pass all validation checks, ensuring they integrate seamlessly with the existing development workflow.

This should significantly improve the efficiency and success rate of future Copilot agent contributions to the repository.

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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.