Could make as an app just for myself, or could make as a public service.
Considerations if made public
There is a Node library for interacting with Git, but it only supports HTTP remotes.
Seems like getting Git over SSH to work in a lightweight serverless environment is gonna be a PITA, and would probably require WASM. Another option is to use a container-based serverless platform like Google Cloud Run. Alternatively, offload everything to a GitHub Action.