## What changed
- Initialize new chat widgets with the current terminal width.
- Determine whether an added history cell is visible using the active history
render mode and the width available after ambient-pet reservations.
- Saturate diff-summary width calculations so extremely narrow terminals remain
renderable.
## Testing
- Cover width-sensitive history insertion with an ambient pet, raw-output mode,
and a diff summary narrower than its indentation.
GitOrigin-RevId: 82d20900e0637431be7646b4f4aba61810ff74c1