chore: release 0.18.28 (#4182)
<!-- CURSOR_SUMMARY -->
> [!NOTE]
> Preps a release and tweaks CI cache behavior.
>
> - Finalizes version to `0.18.28` (from `-dev5`) and updates
`CHANGELOG.md`
> - CI: adds a disk space cleanup step to
`.github/actions/base-cache/action.yml` to purge `/tmp` and pip cache
when rebuilding the venv (no cache hit)
>
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
ec5b735e9d9e696a51ba990dde2ac184fd66d583. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->
