File based Secret support

What would you like added?

Feature/functionality that specifically supports file based secrets. As it becomes more and more common for secrets to be mounted as files, this would enable greater flexibility in deployment.

A clear and concise description of what you want to happen.
A common pattern is to mount secrets as files/volumes to a pod. Some environments also do not allow Kubernetes secrets, and only allow file based secrets (no secrets mounted/used as environment variables). Being to use ASCP, Vault, Kubernetes secrets mounted as volumes, and other Secret CSI providers; to provide GitHub Auth secrets as files.

Documentation which also addresses this use case would also be nice to have as well.

Why is this needed?

File based secrets are becoming a more common pattern over Environment Variables, and some secret stores can only provide secrets via a CSI driver.

A clear and concise description of any alternative solutions or features you've considered.
I see nothing in values.yaml which would allow me to provide a path to a file for a PAT or similar patterns.
Overriding environment variables is not possible with some secret providers that only use CSI drivers.

Additional context

ASCP

Start by tracing values.yaml and the existing runner authentication configuration that currently expects environment variables. Compare how Kubernetes-mounted files from CSI providers would be represented and identify the documentation scope. Done means the desired file-based GitHub authentication flow, configuration shape, and supported secret providers are clearly defined and documented.