feat: Add Kiro provider for AWS Bedrock Claude models

[ikeda-tomoya-swx]

Summary

Add support for Kiro (AWS CodeWhisperer) as a new provider, enabling access to Claude models via AWS Bedrock using Kiro CLI authentication.

Motivation

Kiro CLI provides a convenient way to access Claude models through AWS Bedrock without requiring direct AWS credentials configuration. This provider would allow opencode users who have Kiro CLI installed to use Claude models seamlessly.

Proposed Solution

• Add a new provider kiro with full LanguageModelV2 SDK implementation
• Use plugin-based authentication that reads tokens from Kiro CLI's SQLite database
• Support streaming responses via AWS Event Stream parsing
• Include support for extended thinking mode

Models to Support

• claude-sonnet-4-5
• claude-opus-4-5
• claude-sonnet-4
• claude-haiku-4-5
• claude-3-7-sonnet

Additional Context

Kiro CLI stores OAuth tokens in ~/.local/share/kiro-cli/data.sqlite3 (Linux/macOS) or %APPDATA%/kiro-cli/data.sqlite3 (Windows).