## Why
Managed new-thread defaults could overwrite model, reasoning effort, and service tier settings from an explicitly selected profile.
## What changed
- Treat effective profile settings as explicit launch choices when applying managed defaults. A profile setting for either `model` or `model_reasoning_effort` opts out of both managed values; `service_tier` remains independent.
- Count a profile setting only when it supplies the highest-precedence active value, so settings shadowed by project configuration do not block defaults.
## Testing
Add regression coverage for profile precedence at startup, session replacement, and background task creation, plus custom-provider profiles, service tiers, unrelated profile settings, and project settings that shadow profiles.
GitOrigin-RevId: 98f036b8b7059bab8598283600dbc6967c71d3e3
aff3e0db94