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/whiteboard.rs

@@ -206,7 +206,7 @@ impl WhiteboardHand {
 
     /// Create with custom canvas size
     pub fn with_size(width: f64, height: f64) -> Self {
-        let mut hand = Self::new();
+        let hand = Self::new();
         let mut state = hand.state.blocking_write();
         state.canvas_width = width;
         state.canvas_height = height;