### Motivation
- Clarify the script purpose by renaming it from `smooth-zlevel` to `smooth-walls` because it only targets wall tiles and the previous name implied broader behavior.

### Description
- Rename the script file to `smooth-walls.lua` and update its usage/error strings to `smooth-walls`.
- Rename the documentation to `docs/smooth-walls.rst` and update the title and usage examples to reference `smooth-walls`.
- Preserve the existing smoothing/undo logic that targets wall tiles and the added logic for removing `SmoothWall` jobs in undo mode.

### Testing
- No automated tests were executed for this change.