How to add/remove files for Context?

[chigkim]

I'm trying out codex -p ollama with devstral, and Codex can communicate with the model properly.
I'm wondering how I can add/remove specific files from context? If I run codex -f, it adds all the files including assets in binary.
Also how do you set the maximum context size? It looks like the default is set to 2000000 which is too big for a local model.
Thanks!