Simplify skipped scenario format with placeholder step

Replace @skip tag + placeholder step with just placeholder step.
Skipped scenarios now only need:
  Scénario: Name
    * Scénario non implémenté

- Remove @skip tags from all 26 feature files
- Add step definition returning 'skipped' for placeholder
- Update GherkinHighlighter to hide placeholder and make
  skipped scenarios non-expandable (no chevron, no click)
- Update documentation with new format

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

features/support/hooks.ts

@@ -14,11 +14,8 @@ Before(async function (this: FestipodWorld, scenario) {
   this.screenSourceContent = '';
   this.currentScreen = null;
 
-  // Mark @pending scenarios as pending
-  const isPending = scenario.pickle.tags.some(tag => tag.name === '@pending');
-  if (isPending) {
-    return 'pending';
-  }
+  // Skipped scenarios use the "* Scénario non implémenté" placeholder step
+  // which returns 'skipped' - no special handling needed in the hook
 });
 
 After(async function (this: FestipodWorld, scenario) {