Per review: the new-skill 'ships as a set' block restated CONTRIBUTING's
frontmatter, anatomy, and eval-count rules instead of pointing at them,
which is the 'don't duplicate, reference' rule this guide champions. Keep
the shape (SKILL.md + eval case + optional scripts) and defer the exact
requirements to CONTRIBUTING.md and skill-anatomy.md so they can't drift.
Add docs/developer-onboarding.md: a guided tour for people working on the
repo itself (the five layers, local setup, the verification loop, the
contribution paths, and a suggested reading order), complementing the
authoritative rules in CONTRIBUTING.md, skill-anatomy.md, and evals/README.md.
Link it from the top of CONTRIBUTING.md as the map to its rulebook.
Joan Leon