This change adds [LeftHook](https://lefthook.dev/) as a GitHub hook management tool alongside [MegaLinter](https://megalinter.io/) to lint all relevant files within the repository.
The domain in question is already covered by a wildcard defined further in the list. In AdGuard Home this was causing said domain to have a round-robin DNS entry with the same IP twice.
This change addresses some general housecleaning for CDN hosts:
* Removing all hosts that do not resolve
* Only including the Steam CDN trigger address
