When edit_files receives multiple files, each file was processed
independently: read, compute edits, write. If file B failed, file A
was already written to disk. The caller got an error but had no way
to know which files were modified.
Split editFile into prepareFileEdit (read + compute, no side
effects) and a write phase. The handler runs all preparations
first and writes only if every file's edits succeed.
A write-phase failure (e.g. disk full) can still leave earlier
files committed. True cross-file atomicity would require
filesystem transactions. The prepare phase catches the common
failure modes: bad paths, search misses, permission errors.
Mathias Fredriksson
798a6673c6