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")
• Top-level trigger key: Atom.to_string(trigger.name) (e.g. :deploy -> "deploy")
• Nested trigger key (inside a group): same scheme as a step, "#{group_key}-#{trigger.name}" (e.g. "deploy-rollout")

Also resolves step depends_on atoms/tuples to sibling step key strings. Trigger depends_on (top-level or nested) stays as atoms and is resolved at runtime by Pipette.run/2 against the group key map.