feat: Add SDK package for programmatic MCPHost usage

[ezynda3]

Summary

• Adds SDK package that allows Go applications to use MCPHost programmatically without spawning OS processes
• Maintains identical behavior to CLI including configuration loading, environment variables, and defaults

Changes

• Export required functions from cmd package: Made InitConfig() and LoadConfigWithEnvSubstitution() public to allow SDK to reuse CLI initialization logic
• Create SDK package with:
  • Main MCPHost type for programmatic access
  • Configuration options for overriding defaults
  • Session management capabilities
  • Type helpers for Message and ToolCall
  • Comprehensive documentation and examples
  • Unit test coverage

Key Features

• ✅ Simple Prompt() method for basic usage
• ✅ PromptWithCallbacks() for monitoring tool execution in real-time
• ✅ Session persistence (load/save/clear)
• ✅ Full compatibility with CLI configuration system
• ✅ Respects all environment variables
• ✅ Resource cleanup with Close() method

Testing

• All existing tests pass with race detection
• New SDK tests verify initialization, options, and session management
• Examples compile and run successfully
• Code passes linting and formatting checks

Usage Example

package main

import (
    "context"
    "fmt"
    "github.com/mark3labs/mcphost/sdk"
)

func main() {
    ctx := context.Background()
    
    // Create MCPHost instance
    host, err := sdk.New(ctx, nil)
    if err != nil {
        panic(err)
    }
    defer host.Close()
    
    // Send prompt
    response, err := host.Prompt(ctx, "What is 2+2?")
    fmt.Println(response)
}

Documentation

• Comprehensive README in sdk/README.md
• Working examples in sdk/examples/
• Full API documentation with all methods and types