Enable OpenCode Zen for any opencode/-prefixed model

Routes any model id with the 'opencode/' prefix through the OpenCode
Zen direct provider (strips the prefix before forwarding upstream),
replacing the disabled-state rejection. Priced models still resolve
through OPENCODE_ZEN_MODELS for billing; routing no longer depends on
the priced lookup.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>