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>

src/views/todo-item-component.ts

@@ -4,6 +4,7 @@ import type { TodoItem } from '../core/types';
 export interface TodoItemCallbacks {
 	onToggle: (todo: TodoItem) => void;
 	onMoveClick: (todo: TodoItem) => void;
+	onClick: (todo: TodoItem) => void;
 	onDragStart: (evt: DragEvent, todo: TodoItem) => void;
 	onDragEnd: (evt: DragEvent) => void;
 }
@@ -40,12 +41,16 @@ export function createTodoItemEl(
 		callbacks.onToggle(todo);
 	});
 
-	// Text content
+	// Text content — click to navigate to editor
 	const textEl = itemEl.createEl('span', { cls: 'todo-tracker-item-text' });
 	textEl.setText(todo.text);
 	if (todo.completed) {
 		textEl.addClass('todo-tracker-item-completed');
 	}
+	textEl.addEventListener('click', (evt) => {
+		evt.stopPropagation();
+		callbacks.onClick(todo);
+	});
 
 	// Right-click context menu
 	itemEl.addEventListener('contextmenu', (evt) => {
@@ -65,12 +70,14 @@ export function createTodoItemEl(
 		menu.showAtMouseEvent(evt);
 	});
 
-	// Drag events
+	// Drag events — stopPropagation so nested children don't bubble to parent
 	itemEl.addEventListener('dragstart', (evt) => {
+		evt.stopPropagation();
 		callbacks.onDragStart(evt, todo);
 	});
 
 	itemEl.addEventListener('dragend', (evt) => {
+		evt.stopPropagation();
 		callbacks.onDragEnd(evt);
 	});