refactor: change default model for KiroProvider to auto

[ComicShrimp]

This PR add the following changes:

• Change default model to auto for best cost/performance ratio
• Add test for KiroProvider
• Add KiroProvider in README.md

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Note

Medium Risk
Changes the default model used by KiroProvider, which can alter runtime behavior/cost and may affect users relying on the previous explicit model default. Otherwise limited to documentation formatting and added tests.

Overview
Updates KiroProvider to default to auto model selection (instead of a fixed Claude Sonnet model), aligning with Kiro CLI’s recommended auto-routing.

Adds unit tests covering KiroProvider’s command construction and default model, and updates the README/README header to document KiroProvider (plus minor markdown table formatting cleanups).

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