refactor(skill-lint): export only functions, keep policy state private

Per review: the policy collections (REQUIRED_SECTIONS, SECTION_EXEMPT_SKILLS,
SKILL_REF_PATTERNS, regexes) were exported by reference, so a consumer could
mutate shared state and change lint results process-wide. Export only the
linting functions; keep the policy collections private. No behavior change
(validator output byte-identical).

scripts/lib/skill-lint.js

@@ -217,13 +217,11 @@ function lintSkill(dirName, skillsDir, knownSkills) {
   return lintSkillContent(dirName, content, knownSkills);
 }
 
+// Export only the linting functions. The policy collections (REQUIRED_SECTIONS,
+// SECTION_EXEMPT_SKILLS, SKILL_REF_PATTERNS, and the regexes) stay private so a
+// test or future consumer cannot mutate shared state and change lint results for
+// the rest of the process. Exercise the rules through these functions.
 module.exports = {
-  MAX_DESCRIPTION_LENGTH,
-  KEBAB_CASE,
-  DESCRIPTION_TRIGGER,
-  REQUIRED_SECTIONS,
-  SECTION_EXEMPT_SKILLS,
-  SKILL_REF_PATTERNS,
   parseFrontmatter,
   extractSkillReferences,
   lintSkillContent,