Add comprehensive GitHub Copilot instructions for AutoRest PowerShell development

[Copilot]

This PR adds comprehensive GitHub Copilot instructions to help developers work effectively with the AutoRest PowerShell codebase. The instructions provide a complete guide for setting up, building, testing, and validating changes in this monorepo.

Key Features

Complete Setup Guide

• Exact commands for installing Node.js, Rush.js, AutoRest, and verifying dependencies
• Validated support for Node.js 14.0.0-21.0.0 (tested with 20.19.5)
• PowerShell 6.0+ and .NET SDK 2.0+ requirements

Build and Test Workflows

• rush sync-versions && rush update && rush rebuild - complete bootstrap (~28 seconds total)
• npm test in powershell package - unit tests (3 tests, ~1 second)
• npm run eslint - code quality validation (instant)
• Comprehensive SDK validation test - 52 test cases (~4 minutes)

Validation Requirements

• Manual testing of generated PowerShell modules
• AutoRest generation testing with sample specifications
• End-to-end validation scenarios for CRUD operations and complex models
• Proper timeout specifications to prevent premature cancellation

Developer Experience

• Watch mode for continuous compilation
• Debugging setup with VS Code integration
• Local AutoRest testing with --use:./powershell
• Repository structure and key project locations

Validation

All commands in the instructions have been exhaustively tested:

• ✅ Bootstrap process completes successfully
• ✅ All 52 SDK support test cases pass
• ✅ Generated PowerShell modules import and cmdlets work correctly
• ✅ ESLint passes without warnings
• ✅ AutoRest generation produces 180 files from sample specification

The instructions follow the imperative tone requirement and include explicit "NEVER CANCEL" warnings with specific timeout recommendations for long-running operations.

Fixes #1515.

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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.

[Copilot]

@dolauli 👋 This repository doesn't have Copilot instructions. With Copilot instructions, I can understand the repository better, work faster and produce higher quality PRs.

I can generate a .github/copilot-instructions.md file for you automatically. Click here to open a pre-filled issue and assign it to me. I'll write the instructions, and then tag you for review.