macOS apps should be signed by the developer, not the end user

The README contains several pieces of information that are incorrect.

- [Security warnings don't happen because things aren't installed from the app store](https://github.com/aws-samples/bedrock-engineer/blob/main/README.md?plain=1#L58), they happen because [you haven't followed basic procedure](https://developer.apple.com/help/account/)
- [End users do not codesign](https://github.com/aws-samples/bedrock-engineer/blob/main/README.md?plain=1#L62); [developers do](https://developer.apple.com/documentation/xcode/packaging-mac-software-for-distribution#Build-an-Installer-package)

AWS can afford to properly sign packages. This is a very serious issue you're passing onto your end users.

Start in README.md around lines 58–62 and review the linked Apple account and Xcode packaging guidance. Confirm whether the requested change is documentation-only or also requires the macOS packaging workflow; done means the README accurately describes developer signing and the distributed app or package is signed by the developer.

Written by the indexing model from the issue text.