Enhance twoliter update to add an AUTOGENERATED comment at the top of Twoliter.lock files

We should add a disclaimer when generating the Twoliter.lock file that this file is AUTOGENERATED by twoliter and should not be manually updated but updated through using twoliter update.

Something like:

```toml
# This file is AUTOGENERATED by Twoliter and should only be mutated by running twoliter update
```

Start at the `twoliter update` entry point that generates `Twoliter.lock` files. Confirm that generated files begin with the requested AUTOGENERATED comment and that the resulting content remains valid TOML.

Written by the indexing model from the issue text.