checkpoint: name ephemeral writes by scope (Step / TaskStep)

Mirror the persistent union's verb-free, scope-based naming on the ephemeral side. The store method is already Write, so the request noun should say what is written, not repeat the verb: WriteCheckpoint -> Step (a session step) and WriteTask -> TaskStep (a subagent task step). Both still produce an ephemeral shadow-branch checkpoint; the names match the strategy's SaveStep / SaveTaskStep vocabulary. Bare nouns like checkpoint.Checkpoint were rejected — they stutter on the package's own central domain noun and read as a distinct kind when both requests write the same artifact from different sources.