GitHub Advanced Security trials on Team platform [GA]

### Value Prop
Team plan customers can now try GitHub Advanced Security with a self-serve trial before making a purchase decision. This gives Team customers hands-on access to security capabilities like code scanning, secret scanning, and dependency review to evaluate the real-world impact on their codebase. There's no commitment required to get started — just a direct path to understanding whether GitHub Advanced Security is right for your team.

### Expected Outcome
Customers on the Team plan can now experience GitHub Advanced Security firsthand, removing the guesswork from the buying process. Teams can make confident, informed decisions about investing in advanced security tooling.

The issue describes a planned GitHub Advanced Security trial for Team customers but names no repository files, tests, or implementation entry points. Clarify which codebase and acceptance criteria are in scope before starting; done should include a defined, testable trial experience for the stated security capabilities.

索引モデルが issue の本文から書いたものです。