Create Gemini Agent

Create a named custom agent on the Gemini side.

Example:

curl -X POST "http://localhost:4000/v1beta/agents" \
    -H "Authorization: Bearer sk-..." \
    -H "Content-Type: application/json" \
    -d '{
        "name": "my-custom-slides-agent",
        "base_agent": "waverunner",
        "instructions": "You are a helpful assistant that creates slides.",
        "base_environment": {
            "type": "remote",
            "sources": [
                {"type": "gcs", "source": "gs://eap-templates/slides-skill",
                 "target": "/.agents/skills/slides-skill"}
            ]
        }
    }'