respect model id in opencode.json - #2833

When using Ctrl+T to cycle through model variants (high/middle/low), I'm unclear on:

1. What's the difference between these variants?
2. In what scenarios should I use each variant?
3. Are there any best practices or guidelines for when to switch to high vs middle vs low?
4. I noticed the variant changes in the status bar but don't see clear differences in behavior - is this expected?
   I'm looking for official recommendations or community best practices for using this feature effectively.