[Bug] Auto-compact triggers at 12% context threshold causing infinite loop and rate limit exhaustion

**Bug Description**
Feedback sent (receipt a5a004cf-b8ad-4e18-ab84-5d75688e5783) Sonnet 5 auto-compact fires @ 12% of session context creating a auto compact loop after 1 prompt which has maxxed my session & weekly limit.

**Environment Info**
- Platform: darwin
- Terminal: iTerm.app
- Version: 2.1.272
- Feedback ID: e95540a5-c14e-4476-920c-c8e89e42f00b

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

No source file or test is identified. Start by reproducing the report with Sonnet 5 on darwin in iTerm.app, using the supplied feedback and receipt IDs to trace the auto-compact behavior. Done means auto-compact does not loop after one prompt or exhaust the session and weekly limits.

Written by the indexing model from the issue text.