Compile-time transformer that derives Buildkite key strings for groups, steps, and triggers.

• Group key: Atom.to_string(group.name) (e.g. :api -> "api")
• Step key: "#{group_key}-#{step.name}" (e.g. "api-test")
• Trigger key: Atom.to_string(trigger.name) (e.g. :deploy -> "deploy")

Also resolves depends_on atoms/tuples to their key strings.