Add AI-powered CI check that verifies that ARCHITECTURE is kept up to date

[jlebon]

(Or perhaps it should be a periodic action instead? Not sure.)

But essentially, the idea is that the ARCHITECTURE doc should be a living document and not become fossilized with outdated info. This is nice for humans, but also that doc provides incredibly good grounding for AI agents working on this repo.

[jlebon]

This more generally evolves into arbitrary AI-powered workflows.

https://github.com/google-github-actions/run-gemini-cli/blob/main/docs/authentication.md looks like an easy way to get started once we have the API access bits figured out.