Add Opus 4.6 to Github Copilot

[andrewbenton]

Description

Opus 4.5 and 4.7 for copilot are already available. Only 4.6 is missing for copilot.

[andrewbenton]

I did take a look at running ./cmd/copilot to pick up the new model data, but I think that's org-specific, so I'm not able to see all of the models and settings that we would expect from github's settings page. From what I am able to see, it looks like opus 4.6 via copilot has the same settings as opus 4.5 via copilot.

[andrewbenton]

This could be resolved by #273