Add support for remote builds / architectures to the skill

Consider adding guidance for building on gh runners (extracting the secret and inserting it into the project, either just prompting the user or using a tool such as gh), and change default settings so it would support multiple architectures in the sdkcraft,yaml

Start with sdkcraft.yaml and the repository's existing skill guidance, then trace how build settings and credentials are currently described. Define the remote-build flow for GitHub runners and the intended multi-architecture defaults; the work is done when both are documented or configured clearly enough to use.

Written by the indexing model from the issue text.