Expose GitHub Context out of tag mode

@ashwin-ant @km-anthropic
I’d like to use the GitHub context that’s currently structured in tag mode also in agent mode. I know it can be done using the method described in custom-automations, but reproducing exactly what tag mode does is quite difficult. Another challenge is that the property for determining hidden comments can’t be retrieved via the REST API.

Related to #492, what do you think about the approach of having the context that tag mode builds set as environment variables, and then making it available to the prompt (so it can be used in bash)? If we agree on this direction, I can take care of the implementation.