fix: provider hook skipped for non-catalog providers

Description

When a plugin registers a ProviderHook targeting a provider not in the models-dev catalog (e.g., cursor via opencode-cursor-oauth), the hook is silently skipped because database[providerID] is undefined at provider.ts:1375.

The catalog builder only iterates providers that already exist in the built-in catalog, so third-party plugins can never populate models for custom providers.

Steps to Reproduce

1. Install opencode-cursor-oauth plugin
2. Configure cursor provider in opencode.json
3. Run opencode auth login --provider cursor
4. Start opencode — cursor models do not appear in the model picker

Expected Behavior

Cursor models should appear in the model picker when the plugin is configured.

Actual Behavior

Cursor models are silently skipped because database["cursor"] is undefined.

Root Cause

In packages/opencode/src/provider/provider.ts:1375:

const provider = database[providerID]
if (!provider) continue

The database map is built from the models-dev catalog, which does not include third-party providers like cursor. The plugin hook is skipped before it can populate models.

Fix

Create a skeleton provider entry when database[providerID] is undefined, so the plugins models() function can populate it.