Add comprehensive API features documentation

[codegen-sh]

This PR adds a comprehensive API features documentation (API_FEATURES.md) that explains the Grainchain interface, snapshotting capabilities, and custom Docker support as requested in CG-18637.

📋 What's Included

Core API Interface Documentation

• Unified Sandbox class - Main interface for all sandbox operations
• Core methods - execute(), file operations, state management
• Configuration options - Resource limits, environment variables, provider-specific settings
• Type-safe examples - Comprehensive code examples with proper typing

Snapshotting Capabilities Deep Dive

• Memory vs Filesystem Snapshotting - Clear explanation of the differences
  • Memory snapshots (Morph): Instant <250ms snapshots preserving running processes
  • Filesystem snapshots (E2B, Local, Daytona, Modal): Portable environment state
• Provider-specific support matrix - Which providers support what features
• Advanced workflows - Snapshot chains, terminate/wake-up cycles

Custom Docker Support Analysis

• Current state across all providers:
  • ✅ E2B: Full custom Docker via e2b.Dockerfile
  • ✅ Morph: Custom base images via image_id
  • ✅ Modal: Custom images via programmatic building
  • ❌ Local: No Docker (runs directly on host)
  • 🚧 Docker provider: Coming soon
• Configuration examples for each provider
• Dockerfile examples and template creation

Provider Comparison & Selection Guide

• Feature support matrix - Complete comparison of capabilities
• Performance characteristics - Startup times, snapshot speeds
• Decision tree - Help users choose the right provider
• Use case recommendations - CI/CD, development, production scenarios

🎯 Key Insights

1. Snapshotting is not uniform - Morph offers unique memory snapshots while others do filesystem snapshots
2. Docker support varies significantly - E2B and Modal offer full custom Docker, Morph has curated base images
3. Provider specialization - Each provider is optimized for different use cases
4. Migration is seamless - The unified API allows switching providers with minimal code changes

🔗 Related Issues

• Addresses CG-18637: Explain API features
• Related to CG-18511: Grain chain project context
• Related to CG-18626: Epic improve grain chain benchmarking flow

📁 Files Modified

• API_FEATURES.md - New comprehensive documentation file

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