Skill follow-up: safe-brownfield-refactor

## Summary
Address the follow-up actions for the safe-brownfield-refactor skill as identified in the skills review.

## Acceptance Criteria
- [ ] Add sample characterization test template and a refactor sequencing checklist.

## Scope
- Applies to `skills/safe-brownfield-refactor/`
- Derived from skills-review.md (commit 9c59dbc)

## Parent
- Refs: #297
