refactor: split onboarding provider logic out of bin/lib/onboard.js

[kjw3]

Summary

bin/lib/onboard.js has grown into a large mixed-responsibility module. The current provider selection, validation, and setup logic is working, but the file is now difficult to review and is a hotspot for merge conflicts.

Goal

Reduce the size and change surface of bin/lib/onboard.js without changing onboarding behavior.

Proposed Scope

• Extract provider catalog/config metadata into a dedicated module
• Extract provider validation helpers into a dedicated module
• Extract provider-specific selection/setup flows into strategy-style modules
• Keep onboard.js as the top-level orchestrator for the step flow

Non-Goals

• No behavior changes to onboarding
• No provider UX redesign
• No unrelated cleanup in other onboarding steps

Why

• Lower review complexity
• Reduce rebase/merge conflict pressure in a frequently edited file
• Make provider-specific changes easier to test in isolation
• Improve maintainability for future provider additions

Notes

This is follow-on work from PR #648, where the file size concern was called out during review.

[wscurran]

✨ Thanks for submitting this proposed refactor, it could improve the organization and maintainability of our onboarding code.