Extract onboarding provider selection and review application

> [!IMPORTANT]
> Reserved for @cv. Do not self-assign this issue. Do not open an implementation PR unless @cv requests help.

Parent: #7695

Dependency resolved: #9037 merged in `af1e3a52`.

Status: Provider extraction is queued after the current gateway PR.

## Problem

`src/lib/onboard.ts` owns inference-provider selection and configuration review decisions. Provider extraction must preserve the editable review behavior from #9037.

## Scope

- Move provider selection and review behavior into a feature-owned application module.
- Keep the onboarding entry module responsible for phase sequencing and dependency wiring.
- Reduce the onboarding fan-out and root-file budgets.

## Acceptance criteria

- The provider phase has a named input and result contract.
- The onboarding entry module contains no provider selection or review decision.
- Resume and failure behavior remain unchanged.
- Provider and onboarding tests pass.
- Architecture checks prevent new provider decisions in the entry module.