This PR adds the total and per-file number of lines added and removed
per thread, using the same logic to compute these as in the commit view.
Here's how they show up in the UI:

<img width="500" height="514" alt="Screenshot 2026-01-09 at 11  27@2x"
src="https://github.com/user-attachments/assets/e3a4a14f-a4ee-4e95-a77b-86d14914e9d6"
/>

Release Notes:

- Agent: Added the total and per-file number of lines added and removed
in a thread.