feat(hands): implement 4 new Hands and fix BrowserHand registration

- Add ResearcherHand: DuckDuckGo search, web fetch, report generation
- Add CollectorHand: data collection, aggregation, multiple output formats
- Add ClipHand: video processing (trim, convert, thumbnail, concat)
- Add TwitterHand: Twitter/X automation (tweet, retweet, like, search)
- Fix BrowserHand not registered in Kernel (critical bug)
- Add HandError variant to ZclawError enum
- Update documentation: 9/11 Hands implemented (82%)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

crates/zclaw-hands/src/hands/quiz.rs

@@ -543,7 +543,7 @@ impl QuizHand {
 
                 match quiz {
                     Some(quiz) => {
-                        let score = current_score.unwrap_or(0.0);
+                        let _score = current_score.unwrap_or(0.0);
                         let next_idx = state.current_question_index + 1;
 
                         if next_idx < quiz.questions.len() {