[Adapter] Google Gemini CLI

[shreyas-lyzr]

Summary

Add an adapter for Google Gemini CLI, enabling export, run, and import support.

Details

Gemini CLI is Google's open-source AI agent for the terminal. It uses:

• GEMINI.md at the project root (or ~/.gemini/GEMINI.md globally) for custom instructions
• .gemini/settings.json for model configuration and tool permissions
• Supports Gemini models via Google AI Studio or Vertex AI

A gitagent adapter would allow users to:

• Export: Convert a gitagent agent definition to Gemini CLI format (GEMINI.md + settings)
• Run: Execute a gitagent-defined agent using the gemini CLI runtime
• Import: Convert an existing Gemini CLI project to gitagent format

Acceptance Criteria

• gitagent export --format gemini generates valid GEMINI.md and .gemini/settings.json
• gitagent run ./agent --adapter gemini executes the agent via gemini CLI
• gitagent import --from gemini <path> reads GEMINI.md and produces agent.yaml + SOUL.md
• Documentation added to README adapter table
• Mapping document: what gitagent fields map, what gets dropped, what needs manual setup

Resources

• Gemini CLI repo
• Existing adapters in src/adapters/ for reference (OpenCode and Claude Code adapters are closest patterns)