Add comprehensive GitHub Copilot configuration and prompt templates for Flutter development

[Copilot]

This PR introduces a complete GitHub Copilot configuration system tailored for this Flutter application template, providing developers with structured guidance that aligns with the project's established MVVM + Clean Architecture patterns.

What's Added

Core Configuration Files

• .github/copilot-instructions.md - Comprehensive project-specific guidelines covering architecture patterns, code style, and Flutter best practices
• .github/copilot-workspace.yml - Workspace configuration for enhanced Copilot suggestions with project-specific settings

Prompt Templates (.github/prompt-templates/)

Seven detailed templates covering common development scenarios:

1. Feature Development - Complete MVVM feature implementation with data models, repositories, services, ViewModels, and UI components
2. Bug Fix - Systematic debugging approach with layer-specific strategies and error reproduction techniques
3. Refactoring - Safe code restructuring while preserving functionality and following architecture patterns
4. Testing - Comprehensive testing strategies including unit, widget, and integration tests with mocking
5. UI Component - Reusable component creation with accessibility, theming, and performance considerations
6. API Integration - REST API integration using Retrofit pattern with error handling and caching
7. Performance Optimization - UI rendering, memory management, and app responsiveness improvements

Enhanced Developer Experience

• Updated PR template to track Copilot template usage and architecture compliance
• Comprehensive README with usage instructions and best practices
• Architecture compliance checklists for quality assurance

Key Features

The templates are designed to:

• Maintain consistency with existing codebase patterns (GetIt DI, RxDart streams, Retrofit)
• Provide actionable guidance with copy-paste code examples
• Cover complete workflows from initial implementation to testing and deployment
• Include performance considerations and common pitfall avoidance
• Support all development phases from feature creation to optimization

Example Usage

Developers can now use these templates with GitHub Copilot:

// Reference in comments for contextual help
// Using GitHub Copilot Feature Development Template
// Create a user service following the repository pattern

// Or in Copilot chat
@workspace Using the API Integration template, help me create a repository 
for managing user data with proper error handling and caching.

Benefits

• Faster development with structured guidance for common tasks
• Consistent code quality following established architecture patterns
• Reduced onboarding time for new developers
• Better Copilot suggestions with project-specific context
• Comprehensive testing approach with template-guided test creation

The templates contain over 7,000 lines of detailed guidance, code examples, and best practices specifically tailored to this Flutter template's architecture and development patterns.

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