Add `abc delete` subcommand

### TL;DR

An `abc delete` subcommand would allow all the files belonging to a template to be removed, without affecting neighboring files.

Approximate algorithm: for each file in the manifest, if the file hash matches the recorded hash in the manifest, delete it. If the hash doesn't match, then it has been locally modified, so raise a merge conflict. We'd also have a `--force` flag or something to delete in spite of merge conflicts.

### Detailed design

_No response_

### Alternatives considered

_No response_

### Additional information

_No response_