Add explicit challenges.yml migration pipeline

[minoneer]

Summary

now has an explicit migration pipeline, but still relies on special-case converters and implicit evolution.

Goal

Introduce an explicit versioned migration pipeline for similar in spirit to the one now used for .

Scope

• add versioned migrations for
• create backups before mutation
• replace one-off challenge converters where appropriate
• keep challenge schema evolution explicit and testable

Notes

This is the remaining part split out from #117 after the migration work was completed for 3.4.0.