Skill follow-up: testing-strategy-dotnet

## Summary
Address the follow-up actions for the testing-strategy-dotnet skill as identified in the skills review.

## Acceptance Criteria
- [ ] Add example solution structure and sample CI matrix.

## Scope
- Applies to `skills/testing-strategy-dotnet/`
- Derived from skills-review.md (commit 9c59dbc)

## Parent
- Refs: #297
