## Why
Model requirements must be evaluated against the permissions that will apply
after a settings update. Validating against the current profile can reject a
valid model-and-permission update or allow a later permission-only update to
invalidate the selected model.
## What changed
- Group model, reasoning, service-tier, personality, and approval inputs into
`StepSettings`, with a single apply-and-validate path.
- Apply permission and environment changes before validating step settings, so
auto-review requirements use the proposed effective permissions.
- Revalidate existing step settings when permission or environment changes alter
their constraints, and commit the update only when the complete candidate is
valid.
## Testing
Added unit and integration coverage for atomic model/permission updates,
permission-only revalidation, environment-derived constraints, and both
standalone and turn-start settings operations.
GitOrigin-RevId: f50dfbd300484a09acc51ee4607278f300ca1cd9
