Add a `validate-protocol` skill

scripts/validate-protocol.R reports what's wrong. It doesn't say what to do about it, and most of the contributors arriving aren't R users.

A skill should run it, read the errors, and fix what's mechanically fixable: a missing ## Materials heading, a date that doesn't match the top History entry, a name that isn't kebab-case, a frontmatter field under a retired name. Where the fix is a judgement rather than a correction — which paper belongs in protocol_citation, whether the protocol originates a method at all — it should say what the choice is and stop, not guess.

Distinct from the test-protocol harness in #3: this checks that the document conforms to the standard, not that an agent executing it produces the right answer.

Lives at skills/validate-protocol. Pairs with #35 and #36.

Start by reading scripts/validate-protocol.R and inspect the existing skills/validate-protocol directory. Add a skill that runs the validator, applies only mechanical corrections such as headings, dates, names, and retired frontmatter fields, and reports judgment calls without guessing. It is done when the skill distinguishes fixable errors from choices and remains distinct from the test-protocol harness in #3.