Expand description
Implementation of the UndoEdit tool.
Reverts a file to the content it had before the last FileWriteOrEdit /
MultiFileEdit in this session, using the in-memory checkpoint those tools
record (see bash_state::EditCheckpoint). Per-file LIFO: repeated undos on
one file walk its edits back. A brand-new file’s creation has no prior content
and is not undoable.