Add comprehensive GitHub Copilot instructions for text-cleanup repository

[Copilot]

Created .github/copilot-instructions.md with comprehensive instructions for GitHub Copilot coding agents working in the text-cleanup repository. The instructions provide everything needed to work effectively with this static HTML/CSS/JavaScript web application.

What's Included

Setup and Development Commands:

• Exact bootstrap commands (npm install - 2 seconds to install http-server)
• Multiple development server options (http-server primary, Python alternative)
• No build step required - static site with immediate changes

Critical Validation Requirements:

• Mandatory complete user workflow testing after ANY changes
• Specific test scenarios: text processing, copy functionality, clear functionality
• Manual browser testing requirements with exact steps

Repository Knowledge:

• Complete file structure with purpose explanations
• Code architecture details (vanilla JS, client-side processing only)
• Deployment information (Vercel, static hosting options)
• Common debugging scenarios and solutions

Timing Expectations:

• All operations are near-instantaneous (no build/compilation required)
• Server startup: ~1 second
• Application changes: immediate after browser refresh

The instructions follow the imperative tone requirement and start with a firm directive to reference these instructions first before searching or using bash commands. All commands and workflows have been thoroughly validated to ensure they work correctly every time.

Fixes #4.

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