Add comprehensive GitHub Copilot instructions for VS Code Live Sass Compiler

[Copilot]

This PR adds a comprehensive .github/copilot-instructions.md file that provides GitHub Copilot coding agents with detailed instructions on how to work effectively in the vscode-live-sass-compiler codebase.

What's Included

The instructions provide exhaustive guidance covering:

• Build System: Complete command sequences with accurate timing measurements and proper timeout values
• Development Workflow: Step-by-step processes for making changes, testing, and validation
• Repository Navigation: Key directories, files, and their purposes
• SASS Compilation Testing: Direct validation scenarios using sass-embedded
• Dependency Management: Understanding of npm, TypeScript, Rollup, and extension dependencies
• Troubleshooting: Common issues and their solutions

Key Features

Imperative Tone: All instructions use clear commands like "Run X", "Do not do Y" for immediate action.

Timeout Safety: Explicit warnings about build timing with "NEVER CANCEL" directives and 60+ second timeout recommendations for long-running operations.

Comprehensive Validation: Every command and workflow has been tested and validated to ensure accuracy.

Practical Examples: Real SASS compilation scenarios demonstrating variables, nesting, conditionals, and media queries.

Validated Commands

All build commands have been thoroughly tested:

• npm ci (~14 seconds) - Dependency installation
• npm run lint (~1.5 seconds) - ESLint validation
• npm run rollup (~15 seconds) - Production build
• npm run vscode:prepublish (~16 seconds) - Full build pipeline
• npm run pretest (~21 seconds) - Test environment preparation

The instructions also include direct SASS compilation testing using npx sass-embedded to validate extension functionality without requiring a full VS Code environment.

These instructions will help GitHub Copilot agents navigate the codebase efficiently, understand the build process, and make changes with confidence while avoiding common pitfalls.

Fixes #419.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• update.code.visualstudio.com

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

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✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.