4 `.mdc` files had unquoted globs values containing `*` (YAML alias
indicator) and 1 had unquoted description containing `: ` (YAML
mapping indicator), making them invalid YAML. Standard parsers like
gray-matter fail on these.
Also adds a validation script (`validate:rules`) using gray-matter
to catch invalid frontmatter in CI.
Rune Botten
730c6f8ab5