allow to output report json to stdout

currently it goes to `--output` directory however in some cases it might be easier to go to stdout or specific file to enable flows like:

```
github-analyzer --json | jq ...
```

Start by tracing the github-analyzer --json entry point and how it currently uses --output. Run the command with a sample report and inspect the existing output behavior; done means JSON can be streamed into jq and can also be directed to a specific file without breaking directory output.

索引モデルが issue の本文から書いたものです。