✨ Set up GitHub Copilot instructions for scrape-cli

[Copilot]

This PR adds comprehensive GitHub Copilot instructions to help developers and AI assistants better understand the scrape-cli project structure, coding patterns, and development guidelines.

What's Added

Created .github/copilot-instructions.md with detailed documentation covering:

• Project Overview: CLI tool for HTML element extraction using XPath/CSS selectors
• Architecture & Dependencies: Core structure with lxml, cssselect, and requests
• Coding Patterns: Command-line interface, HTML processing pipeline, error handling
• Key Functions: Detailed explanations of is_xpath(), convert_css_to_xpath(), clean_text(), and detect_charset()
• Development Guidelines: Testing approaches, code style, common tasks
• Integration Patterns: Unix pipeline-friendly design examples

Benefits

The instructions will help GitHub Copilot provide:

• Better context-aware code suggestions
• Understanding of the project's specific patterns and conventions
• Knowledge of the HTML processing pipeline and selector handling
• Awareness of CLI argument validation and error handling patterns

Example of patterns documented:

# XPath detection patterns
def is_xpath(expression):
    """Check for XPath patterns like //, ::, [], @, functions"""
    
# CSS to XPath conversion with error handling
def convert_css_to_xpath(expression):
    """Convert CSS selectors using cssselect.GenericTranslator"""

This enhances the development experience by providing AI assistants with comprehensive project context.

Fixes #24.

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