feat: Add refactored Go SDK with comprehensive tests and documentation

This commit introduces a professional, production-ready Go SDK for Shifu
IoT device management with significant architectural improvements.

Features:
- ✅ Client-based architecture with dependency injection
- 🔧 Kubernetes EdgeDevice integration
- 📊 Health monitoring with customizable intervals
- 🎯 Flexible configuration via Config struct
- 🧪 78.9% test coverage with comprehensive test suite
- 📚 Complete documentation with examples

Architecture:
- Eliminated global state in favor of Client struct
- Added interfaces (EdgeDeviceClient, ConfigLoader, HealthChecker)
- Support for multiple device management with isolated instances
- Better error handling with wrapped errors
- Type-safe API following Go idioms

API:
- NewClient() and NewClientFromEnv() constructors
- Client.Start() for health monitoring
- Client.GetEdgeDevice() for device information
- Client.UpdatePhase() for status updates
- Client.GetConfigMap() for configuration loading
- Backward compatible deprecated global functions

Testing:
- Comprehensive test suite with mock HTTP servers
- Unit tests for all major functions
- Edge case and error handling tests
- Benchmark tests included

Documentation:
- Professional README.md with quick start guide
- Complete API reference
- Multiple usage examples
- Troubleshooting guide
- Migration guide from deprecated functions

Files:
- sdk.go: Main SDK implementation (402 lines)
- sdk_test.go: Comprehensive test suite (940 lines)
- README.md: Professional documentation (949 lines)
- go.mod: Module dependencies
- go.sum: Dependency checksums

🤖 Generated with Claude Code

Co-Authored-By: Claude <noreply@anthropic.com>

Author: rhoninl