Add a "Last turn" filter to the Changes tab

Add a "Last turn" view to the Changes tab so users can review only the files changed by the agent since their previous message instead of sorting through the full session diff

Start by locating the Changes tab and how it builds the full session diff. Trace how agent messages define turn boundaries, then add a view that limits the displayed files to changes since the previous message. Done means users can select "Last turn" and review only that turn's changed files without affecting the full diff view.

Written by the indexing model from the issue text.