Populate PromptResponse.usage from Codex token usage

[simonrosenberg]

Summary

This wires Codex token usage through to ACP prompt responses instead of dropping it at turn completion.

Specifically:

• return a full PromptResponse from the ACP prompt path
• populate PromptResponse.usage from Codex token counts
• compute PromptResponse.usage as a per-turn delta from cumulative token usage
• fix UsageUpdate.used to report cumulative tokens currently in context, not only the latest API call
• add a regression test covering sequential turns

Resolves #165
Resolves #99

Why

codex-acp currently emits bare PromptResponse::new(stop_reason) responses, so ACP clients never receive per-turn token usage even though upstream Codex already exposes token counts.

This breaks telemetry in ACP clients and benchmarks that rely on PromptResponse.usage.

Scope

This PR intentionally does not populate UsageUpdate.cost.
That remains a separate issue because the adapter does not appear to receive an authoritative cumulative cost signal from upstream Codex today.

Tracked separately in #209.

Testing

Added a targeted regression test for:

• first turn usage population
• second turn delta calculation
• cumulative UsageUpdate.used

I could not run cargo test locally in this environment because cargo is not installed on this machine.

[simonrosenberg]

This PR also resolves #99 — it populates PromptResponse.usage with per-turn token counts from Codex, which is the data requested there.