[Feature Request] Choose a repo when opening plannotator-review in a workspace with multiple repos

I often have my pi agent open in a directory containing multiple semi-related repositories it can access and reference for a specific task. I'd like to use `/plannotator-review` to check the latest commit for the repo i've been working in only, but by default that opens up changes for all repos in the current workspace, and i can't see an option to just select one of them. It'd be cool if I could pass in an argument for a path to `/plannotator-review`

```sh
# e.g. something like this
/plannotator-review ./repoOrFolderIWantToReview

# or this if you want current workspace dir to be default root
/plannotator-review repoOrFolderIWantToReview
```

Start by locating the /plannotator-review command handling and the code that discovers repositories in the current workspace. Add support for an optional repository path, then verify that reviewing with the path only shows changes from the selected repository while the default still uses the workspace.

Written by the indexing model from the issue text.