feat: interactive diff confirmation for writes

[maxmekiska]

Add interactive diff confirmation for file changes (write/edit tools). Users see a colored diff with line numbers and
can accept or decline. Declined changes include optional feedback sent to the model.

UI Example

Summary

- Simple selection menu: [Enter] = accept, [d] = decline
- Colored diff: red (removed), green (added), dim (context)
- Optional decline reason sent back to model as feedback
- No new dependencies > only stdlib: difflib, termios, tty and sys