Update Claude provider comparison to include Kiro Gateway implementation

- Add comprehensive Kiro Gateway analysis alongside Claude Code comparison
- Document Kiro's unified intermediate message format approach
- Compare streaming implementations (SSE vs AWS Event Stream)
- Document Kiro's advanced features: thinking injection, tool content stripping,
  image extraction, truncation recovery, model name normalization
- Add comparison tables for architecture, message handling, tools, streaming
- Identify patterns from Kiro that could improve AISBF (unified format,
  message validation, multimodal support)