Extract onboarding messaging application ownership

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

Parent: #7695

## Problem

`src/lib/onboard.ts` owns messaging-channel setup, credential staging, and resume decisions.

## Scope

- Move messaging-channel onboarding 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 messaging phase has a named input and result contract.
- The onboarding entry module contains no messaging-channel decision.
- Resume and failure behavior remain unchanged.
- Messaging and onboarding tests pass.
- Architecture checks prevent new messaging decisions in the entry module.