Add comprehensive GitHub Copilot support documentation

[Copilot]

This PR adds comprehensive documentation to support GitHub Copilot development workflows for the Datas project. The documentation provides detailed context about the codebase patterns, conventions, and domain-specific knowledge to enable more accurate and consistent code generation.

What's Added

Core Documentation Files

• .copilot-instructions.md - Main context file with project overview, architecture patterns, and coding conventions
• .github/copilot-instructions.md - Development guide with code generation patterns and templates
• CODING_PATTERNS.md - Comprehensive coding standards and best practices
• COPILOT_WORKFLOW.md - Specific workflows and prompt templates for common development tasks

Enhanced Code Intelligence

• Added comprehensive JSDoc documentation to the DateUtils class with parameter types, return values, and usage examples
• Enhanced method documentation for better code completion and understanding
• Maintained 100% compatibility with existing functionality (54 tests still passing)

Project Configuration

• .copilot-config.json - Structured project information for quick reference
• .gitattributes - Language detection and file type configuration

Key Features for GitHub Copilot

Domain-Specific Context

The documentation provides detailed context about:

• Astronomical algorithms for moon phase calculations using Julian day conversion
• Portuguese localization patterns for UI text and error messages
• Date manipulation best practices avoiding common JavaScript pitfalls
• Calendar generation algorithms with proper month boundary handling

Development Patterns

• Consistent naming conventions (calculate*, get*, is*, format*)
• Input validation patterns with Portuguese error messages
• UI interaction patterns with visual feedback (border colors for success/error states)
• Test-driven development workflows using Jest with AAA pattern

Code Generation Templates

Provides ready-to-use templates for:

• Adding new date utility methods
• Creating UI event handlers
• Writing comprehensive Jest tests
• Implementing astronomical calculations

Example Usage

With this documentation, GitHub Copilot can now generate code like:

/**
 * Calcula dias úteis entre duas datas (excluindo fins de semana)
 * @param {string|Date} startDate - Data inicial
 * @param {string|Date} endDate - Data final
 * @returns {number} Número de dias úteis
 */
static businessDaysBetween(startDate, endDate) {
    if (!startDate || !endDate) {
        throw new Error('Ambas as datas são obrigatórias');
    }
    // Implementation following project patterns...
}

The interface remains fully functional with all features working as expected, including the astronomical moon phase calculations and responsive calendar display.

Fixes #13.

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💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.