[Feature Request] Make Opus 4.1 fallback behavior less aggressive

**Bug Description**
the fallback to Opus 4.8 on this is too aggressive. I'm the good guy doing good things about bad guys here! :)

**Environment Info**
- Platform: darwin
- Terminal: Apple_Terminal
- Version: 2.1.263
- Feedback ID: 384e96ee-1feb-46df-95e1-394e257aa0e9

**Errors**
```json
[]
```

The issue names no source files, tests, or entry points. Start by reproducing the fallback from Opus 4.1 to Opus 4.8 on the reported darwin terminal environment, then trace the fallback decision and document the intended threshold with the maintainer. Done means the fallback is no longer triggered as aggressively and the behavior is covered by a reproducible check.

索引モデルが issue の本文から書いたものです。