fix(skill-lint): strip every negated trigger clause, not just the first

The negated-trigger check removed only the first "Do not use when …" clause
before testing whether a positive trigger remained, because the replace used the
non-global DESCRIPTION_TRIGGER_NEGATE. A description carrying two or more negated
clauses and no positive trigger left a second "use when" substring behind, which
DESCRIPTION_TRIGGER then matched, so the skill passed validation with no trigger
at all.

One negated clause was correctly rejected; two were accepted. Adds a global form
of the pattern for the strip and keeps the non-global one for .test(), since a
/g regex carries lastIndex across test() calls.

scripts/lib/skill-lint-test.js

@@ -100,6 +100,20 @@ test('reports a description with no trigger clause', () => {
   assert.match(errors[0], /no 'when to use' trigger/);
 });
 
+test('a description whose only triggers are negated is rejected regardless of how many there are', () => {
+  const content = withAllSections(
+    [
+      '---',
+      'name: alpha',
+      'description: Designs alphas. Do not use when building betas. Never use when the input is JSON.',
+      '---',
+    ].join('\n')
+  );
+  const { errors } = lintSkillContent('alpha', content, KNOWN);
+  assert.equal(errors.length, 1);
+  assert.match(errors[0], /no 'when to use' trigger/);
+});
+
 test('reports frontmatter name that disagrees with the directory', () => {
   const content = withAllSections(
     ['---', 'name: beta', 'description: Designs alphas. Use when building one.', '---'].join('\n')

scripts/lib/skill-lint.js

@@ -38,6 +38,12 @@ const KEBAB_CASE = /^[a-z0-9]+(-[a-z0-9]+)*$/;
 // describe exclusions, not trigger conditions.
 const DESCRIPTION_TRIGGER        = /\buse (this )?when\b|\buse (before|after|during)\b/i;
 const DESCRIPTION_TRIGGER_NEGATE = /\b(do not|don't|never) use (this )?(when|before|after|during)\b/i;
+// Same pattern, global — used to strip *every* negated clause before checking
+// whether a positive trigger remains. Without /g only the first is removed, so
+// a description carrying two or more negated clauses and no positive one would
+// still match DESCRIPTION_TRIGGER on the leftovers and pass. Kept separate from
+// the non-global form above because a /g regex carries lastIndex across .test().
+const DESCRIPTION_TRIGGER_NEGATE_ALL = new RegExp(DESCRIPTION_TRIGGER_NEGATE.source, 'gi');
 
 // Sections every standard SKILL.md must contain.
 // Each entry is an array of acceptable heading strings — the first
@@ -287,7 +293,7 @@ function lintSkillContent(dirName, content, knownSkills) {
     }
     const hasTrigger       = DESCRIPTION_TRIGGER.test(fm.description);
     const onlyNegated      = hasTrigger && DESCRIPTION_TRIGGER_NEGATE.test(fm.description)
-      && !fm.description.replace(DESCRIPTION_TRIGGER_NEGATE, '').match(DESCRIPTION_TRIGGER);
+      && !fm.description.replace(DESCRIPTION_TRIGGER_NEGATE_ALL, '').match(DESCRIPTION_TRIGGER);
     if (!hasTrigger || onlyNegated) {
       errors.push(
         `Description has no 'when to use' trigger — add a "Use when …" clause ` +