Add focus activation, 3-zone drag nesting, and click-to-navigate

Implement remaining Round 3 enhancements:
- ArrowDown when panel unfocused activates it at first item (like Outline view)
- 3-zone drag-drop: top/bottom thirds insert above/below, middle third nests as child
- Click on todo text to focus it in editor (onClick callback)
- Dragging parent automatically moves nested children (stopPropagation fix)
- Cross-file move inserts todo below heading with blank line (addBlankLine param)
- Updated CLAUDE.md with sidebar view architecture documentation

Build: 85 tests pass, production build succeeds.

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

styles.css

@@ -103,6 +103,12 @@
 	border-bottom: 2px solid var(--interactive-accent);
 }
 
+.todo-tracker-drop-nest {
+	outline: 2px solid var(--interactive-accent);
+	outline-offset: -2px;
+	background-color: var(--background-modifier-hover);
+}
+
 .todo-tracker-drag-over {
 	background-color: var(--background-modifier-hover);
 }