Smoother chat composer, cleaner change summaries

When you open the harness, model, or agent picker in the chat composer, it now highlights the option that is currently active instead of jumping to the top of the list — so you can see what you're on at a glance. Pressing Esc now closes an open picker; previously it had no effect.

The "Modified nodes" summary shown after an agent edits your data is now clearly read-only. The version transition (for example v1 → v4) no longer renders as an editable text box — it's just information about what changed, with the Diff and Undo actions beside it.