Show token usage and estimated cost per thread

[stoivo]

I’d like to see how many tokens each thread has consumed and its estimated cost, so I can understand the expense of individual tasks and compare model choices.

Ideally, each thread would have a compact usage summary showing:

• Total token usage, with input, output, and cached tokens where available.
• Estimated API cost, clearly distinguished from actual billed spend for subscription accounts.
• A breakdown by model when multiple models are used in the same thread.

A thread breakdown on the Usage page would also be useful. Maybe link from the thread too.

It has been implemented in #9017 but I want to open a discussion in case PR gets closes or too stale.

It think the UI implemented looks fine.

Related work:

• feat: add usage / quota visibility for Codex sessions and accounts #228 previously requested per-thread usage alongside account quotas.
• feat(usage): add thread and subagent breakdown #9016 proposes thread and subagent breakdowns on the Usage page.
• feat(web): show thread API cost in composer #9136 proposed a thread cost indicator in the composer and was closed pending the V2 rewrite.

Could this issue track the feature request through V2, independently of those implementation PRs?

Also somewhat related to #6683