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)
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