Add support for running opencode github action without using the opencode-hosted github app

[josh-clanton-powerschool]

I'm looking into setting up an opencode github actions workflow, but due to security concerns, I don't believe my company would be willing to install the existing opencode github app.

If there is a way to orchestrate it without the github app, that would be very appealing, but I'm sure we'd be able to host an internal copy of the github app if we had a way to configure opencode to point to our internal version for the oidc dance.

[fwang]

Yeah makes sense.

We can make the GH action accept a user provided token ie. GH_TOKEN with permissions to create PRs and comment. If provided, the GH action would not do the whole token exchange flow w/ the GH app.