chore: kill the dead Polaris Alpha code

[ariane-emory]

The 'Polaris Alpha' model, an early preview snapshot of GPT-5.1, was available briefly on OpenRouter, but has long since given up the ghost and left the land of the living - ne'ermore will we be graced with its presence.

opencode nonetheless retains a dedicated prompt file for this model and a reference thereto.

As this model has left us and will not return to use, it's high time that we bury it and let it rest in peace.

Let's all share a brief moment of silence in honour of this departed model. So long Polaris Alpha, you were... kind of okay, I guess. We won't forget you... aw, who'm I kidding? I'd almost forgotten already until I stumbled onto this prompt file by happenstance, hah! Amen, or whatever.

Changes

• Remove the dedicated prompt file for the dead Polaris Alpha model and a line of dead code by removing a reference thereto.