Generate GitHub Copilot instructions for grunt-plugins monorepo development workflow

[Copilot]

This PR adds comprehensive GitHub Copilot instructions to help developers work effectively in the grunt-plugins monorepo. The instructions document the complete development workflow, including setup, build processes, validation commands, and known limitations.

Key Features

Validated Command Documentation:

• Documented exact timing for all commands (e.g., npm install takes 2-3 minutes)
• Added "NEVER CANCEL" warnings for long-running operations
• Identified working vs broken commands with specific error details

Repository Structure Guidance:

• Complete Rush monorepo layout explanation
• Package dependencies and relationships
• Key configuration files and their purposes

Known Issues Documentation:

• Build failures due to TypeScript/Node.js disposable symbol compatibility
• Missing tslint dependency (deprecated tool)
• Grunt dependency path issues in Rush monorepo structure
• Phantom node_modules warnings from Rush

Alternative Validation Methods:

• External TypeScript validation: npx tsc --noEmit
• External ESLint validation: npx eslint . --ext .ts
• Manual testing approaches for plugin development

Development Workflow:

• Step-by-step setup instructions with timing expectations
• Dependency management with Rush commands
• Workarounds for current build system limitations
• Manual validation scenarios for different development tasks

The instructions prioritize accuracy over optimism - documenting the current state including broken functionality so developers have realistic expectations and can focus on productive workflows.

Fixes #318.

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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.