weichen
e1fac5864d
Why: the completion doc's N2 requires skills validated per the open spec; today frontmatter was a description-only line hack with silent skips, no name field, and no error surface. Evidence: GOWORK=off go vet + go test -count=1 ./... green; parser units cover all field forms and reject malformed shapes with located errors; the validation matrix covers kebab-case/name-equals-dir/ required description/limits; Store.Diagnostics surfaces per-skill reasons and Load returns the located validation error for same-named invalid skills; existing fixtures updated to spec form. Impact: stricter by design — skills without name/description or with unknown frontmatter keys now produce located errors instead of silently loading or silently skipping; ValidateSkillDocument is exported so the product plugin/seed paths share the same validator. Refs: Agent-System-Documents#42 (N2 plan WP4)