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_

Look for the CLI command structure in the codebase, likely in a cmd/ directory. Find where template manifests are stored and how file hashes are recorded. The delete subcommand needs to read the manifest, compare hashes, and handle conflicts. Check for existing subcommands like 'apply' to understand the pattern. Running the existing tests will verify the implementation.

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