Trials' Session traces in the Experiment's Trace, and each Arm's tokens and cost #1023

Parent

#977

What to build

Once Session traces exist (#985), each Trial's and Grader's session joins its Experiment's Trace. The trace ID comes from the Experiment's Command, with the Trials and Graders as spans beneath it. Each Arm's tokens by Model, and its cost where the Harness reports one, are summed from its Trials' Session traces and shown beside its minutes.

Acceptance criteria

• An Experiment's Trace holds its Trials and Graders as spans beneath it.
• thirdshift experiments <id> --json shows each Arm's tokens by Model, and its cost where reported.