Auto register Scion agents in Gemini Enterprise Agent Platform

As Opt-in, I suggest enabling a config to auto register Scion agents in [Gemini Enterprise Agent Platform](https://cloud.google.com/blog/products/ai-machine-learning/introducing-gemini-enterprise-agent-platform?e=48754805)

This can be achieved via [this API](https://docs.cloud.google.com/agent-registry/reference/rest/v1alpha/projects.locations.services#Service)

Start by reviewing Scion's configuration and agent lifecycle to find where an opt-in setting and registration call would fit. Read the linked Gemini Enterprise Agent Platform Service API documentation and determine the required project, location, and service fields. Done means the opt-in path registers Scion agents through that API without changing default behavior.

Written by the indexing model from the issue text.