When `nbd update` is run without `--json`, print a git-diff-style
+/- summary of changed fields instead of the full ticket.
- Add `format_diff` and `print_diff` to display.rs
- Capture `old` ticket snapshot in `cmd_update` before mutations
- Use `print_diff` for the non-JSON branch of `cmd_update`
- Add unit tests (4) and integration tests (3) covering diff output,
JSON fallback, and the no-changes case
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Elijah Voigt