Add Google Gemini adapter

[simonhsze]

What

Add a Google Gemini adapter (adapters/gemini.py) that implements the LLMAdapter interface.

Why

Gemini models are free-tier friendly and widely used. Expanding backend support lowers the barrier to try Agent Constitution.

How

1. Copy adapters/anthropic_api.py as a starting template
2. Implement call() using the google-generativeai Python SDK
3. Map response to LLMResponse
4. Add lazy import in adapters/__init__.py
5. Add tests (no real API calls)
6. Add google-generativeai to optional dependencies in pyproject.toml

Reference

• adapters/base.py — the interface to implement
• adapters/ollama.py — another HTTP-based adapter for reference