[Configuration] Allow directories to be provided instead of searching for backend

### TL;DR

Allow users to provide a list of directories instead of always detecting the backend configuration.

This should allow Guardian to still detect which directories have changed and instead marry that against a list of provided directories.

### Detailed design

```markdown
We should probably introduce a config file at this point to start allowing greater customization to guardians features.
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### Alternatives considered

_No response_

### Additional information

_No response_

The issue proposes a configuration file to allow specifying directories instead of auto-detecting backends. Start by examining the existing backend detection logic in the codebase, likely in a main.go or config package. Look for how directories are currently scanned. 'Done' means users can provide a list of directories via config, and Guardian uses that list instead of searching.

Written by the indexing model from the issue text.