Decompose onboarding into phase composition #7695

Parent: #7691

Problem

src/lib/onboard.ts imports more than 200 runtime modules. It contains dependency wiring and product behavior.

Scope

• Define explicit onboarding phases.
• Move phase behavior into feature-owned application modules.
• Keep the onboarding entry module responsible for sequencing and dependency wiring.
• Reduce the onboarding fan-out and root-file budgets in each PR.

Remaining sequence

1. Extract messaging application ownership (Extract onboarding messaging application ownership #9170). Work is in progress.
2. Extract gateway lifecycle ownership (Extract onboarding gateway lifecycle application #9171). Work is in progress.
3. Extract provider selection and review ownership (Extract onboarding provider selection and review application #9169). This work is queued after the current gateway PR. PR feat(onboard): make configuration review editable #9037 resolved the prior dependency.
4. Enforce the zero-decision entry boundary (Enforce the onboarding entry composition boundary #9172). This work waits for Extract onboarding provider selection and review application #9169.

Acceptance criteria

• Each phase has a named input and result contract.
• The entry module contains no provider, gateway, messaging, or policy decision.
• Resume and failure behavior remain unchanged.
• Onboarding tests pass after each phase move.