v0.4.2: /undo checkpoints

What's new

• /undo: session-scoped checkpoints for file mutations. edit_file and write_file record the pre-mutation bytes on an in-memory stack; /undo in the REPL restores the latest checkpoint (removing files created this session). Per-edit granularity. Bash side effects stay untracked by design.
• README LoC numbers refreshed with the sentinel test staying green (engine 1201 net lines, package 21 files / 2008 physical / 1614 net).

Verification

• 108 tests pass, including the new checkpoint suite (edit-undo restores bytes, write-undo removes the created file, failed edits leave no checkpoint).
• ruff check clean; build + twine check pass.