OpenCode model parsing and command fallback resolution

OpenCode splits model references at the first slash: openrouter/anthropic/claude-opus-4.8 becomes provider openrouter and model anthropic/claude-opus-4.8. Commands may fail while normal chat works because chat carries an already-resolved input.model, whereas commands can fall back to the configured default model. If that default references a provider unavailable in the command-time registry, resolution fails. Verify both the first-slash model reference and that the configured provider is consistently loaded.