feat(providers): route the opencode Go subscription

opencode Go is a flat $10/month plan fronting popular open coding models.
It serves its own endpoint, https://opencode.ai/zen/go/v1, separate from
the pay-per-use Zen one, and the same key works for both.

Adds the provider plus the eight models Go exposes over /chat/completions
whose context windows can be mirrored from the same model already in this
registry. Go's /responses model (gpt-5.6-luna) and its seven /messages
models need the other protocols and are deliberately left out.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>