add Google Gemini LLM provider

[yai-dev]

Summary

Add a native Google Gemini provider to @agentrail/core.

Background

Agentrail currently ships native OpenAI and Anthropic providers. Gemini is the next major provider to support directly rather than only through compatibility layers.

Proposed Implementation

1. Add the chosen Gemini SDK dependency to packages/core/package.json
2. Implement gemini-llm-provider.ts in packages/core/src/llm/providers/
3. Add side-effect registration export(s) through the current providers index
4. Add provider tests using the current core test setup

Acceptance Criteria

• provider: "gemini" works in ModelConfig
• Text streaming works
• Tool calling works
• Gemini-specific reasoning / thinking behavior is mapped where supported
• baseUrl or equivalent endpoint override behavior is documented if supported
• Tests pass
• Changeset included