fix(agent): persist Codex prompt token usage

[tatoalo]

Problem

Codex app-server sessions calculate per-turn token usage, but the adapter returned only the stop reason from prompt(). The agent server therefore had no usage data to persist on the task run, leaving Codex cloud-run token totals empty.

Changes

• Return Codex token usage with the ACP prompt response
• Preserve single ownership for steered prompts to avoid double-counting
• Aggregate usage across plan handoff turns
• Keep the existing PostHog usage notifications compatible
• Extend the existing adapter tests for prompt, steering, plan handoff, and usage tracking

[trunk-io]

😎 Merged successfully - details.

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 45298c3.}

[github-actions]

Well-tested bug fix that properly threads token usage through the prompt response — no showstoppers, all callsites handle the updated undefined-return correctly, and the merging logic for multi-turn plan cycles is sound.

[greptile-apps]

_{Reviews (1): Last reviewed commit: "fix(agent): persist Codex prompt token u..." | Re-trigger Greptile}