feat(tools): include a line diff in edit-file results

Problem: an edit-file result only said how many replacements were made, so
the transcript could not show what changed.
Fix: edit-diff appends, per replacement, up to two context lines ("  "),
the removed lines ("- ") and the added lines ("+ "), capped at 60 lines
with a trailing count. Line-based over the spanned lines only; no LCS.