Hi,

Currently shomei appears to only work when creating a new repository. I attempted to use it with an already existing repository that I created, but encountered an error.

Steps to reproduce:

1. Created a GitHub repository manually
2. Generated a fine-grained personal access token with repository permissions for Contents and Metadata
3. Made this token to be repository specific (connected it directly to the mirror repo)
4. Ran the tool with my GitHub username, same repository name and PAT, yet it could not find the repo in question.

Expected Behavior:
CLI tool should detect if repository exists, or at least ask the user if they want to create new repository or use existing one.

Actual Behavior:
Error message:

!!! couldn't create repo: Resource not accessible by personal access token