Remove Kiro Gateway from Claude provider comparison

Kiro Gateway is an Amazon Q Developer implementation using AWS CodeWhisperer API,
not a Claude provider. The comparison now focuses on actual Claude implementations:

- AISBF Claude Provider (direct HTTP with OAuth2)
- Original Claude Code (TypeScript/React from Anthropic)
- KiloCode (TypeScript using @ai-sdk/anthropic)

Removed all Kiro-related sections including:
- Kiro Gateway architecture comparison
- Kiro message conversion and tool handling
- Kiro streaming (AWS Event Stream)
- Kiro model name normalization
- Kiro exclusive features (thinking injection, truncation recovery, etc.)

Document now cleanly compares three Claude provider implementations.