Build a test environment for the skills repository #8

Five layers, of which only the last costs money: contract (tests/skills/<name>/), unit (tests/template/unit/), integration (tests/template/integration/), repository (tests/repo/), behaviour (claude plugin eval).

The skill is the unit under test, so its tests live with it: one thin file per skill over a shared contract. tests/ holds test cases and nothing else; the harness lives in scripts/testkit/ behind the @testkit alias.

The frontmatter parser is deliberately independent of anything this repository ships to its users: a test that parses a skill with the same code the skill is validated by cannot catch that parser being wrong.

npm run test:prove applies mutations one at a time and fails if the suite stays green.