Add TDD skill (adapted from mattpocock/skills)

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Summary

Add a TDD (Test-Driven Development) skill to the toolkit, adapted from mattpocock/skills/tdd.

Attribution

Original work by Matt Pocock — https://github.com/mattpocock/skills

Content

The skill covers:

• Red-green-refactor loop with vertical slicing (tracer bullets)
• Anti-pattern: horizontal slices (writing all tests first)
• Deep modules (small interface, deep implementation)
• Interface design for testability
• Mocking at system boundaries only
• Refactoring candidates post-TDD cycle
• Good vs bad test examples

Structure

skills/tdd/
  SKILL.md          # Core TDD workflow + philosophy
  references/
    deep-modules.md
    interface-design.md
    mocking.md
    refactoring.md
    tests.md

Plugin home

New plugin: dev-practices — not devops, this is a development practice skill.

Tasks

• Create dev-practices plugin scaffold
• Adapt TDD skill content (rewrite in project voice/conventions)
• Add to marketplace.json
• Bump version

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