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/core/todo-transformer.ts

@@ -137,18 +137,47 @@ export function moveTodoUnderHeadingInFile(
 	return moveTodoWithChildren(content, todoLineNumber, childLineNumbers, targetLineNumber);
 }
 
+/**
+ * Adjust indentation of todo lines by a delta (positive = indent, negative = outdent).
+ * Each level is 2 spaces. Will not outdent past zero.
+ */
+export function indentTodoLines(todoLines: string[], indentDelta: number): string[] {
+	if (indentDelta === 0) return [...todoLines];
+
+	const indent = '  '; // 2 spaces per level
+
+	return todoLines.map((line) => {
+		if (indentDelta > 0) {
+			return indent.repeat(indentDelta) + line;
+		} else {
+			// Outdent: remove leading spaces
+			let result = line;
+			for (let i = 0; i < Math.abs(indentDelta); i++) {
+				if (result.startsWith(indent)) {
+					result = result.slice(indent.length);
+				} else if (result.startsWith('\t')) {
+					result = result.slice(1);
+				}
+			}
+			return result;
+		}
+	});
+}
+
 /**
  * Insert a todo line under a specific heading.
  * @param content - The file content
  * @param headingLineNumber - The line number of the heading (0-based)
  * @param todoLine - The todo line to insert
  * @param nextHeadingLineNumber - Optional line number of the next heading (for section boundary)
+ * @param addBlankLine - Whether to add a blank line between the heading and the todo
  */
 export function insertTodoUnderHeading(
 	content: string,
 	headingLineNumber: number,
 	todoLine: string,
-	nextHeadingLineNumber?: number
+	nextHeadingLineNumber?: number,
+	addBlankLine = false
 ): string {
 	const lines = content.split('\n');
 
@@ -156,15 +185,26 @@ export function insertTodoUnderHeading(
 	let insertPosition: number;
 
 	if (nextHeadingLineNumber !== undefined) {
-		// Insert just before the next heading (at the end of this section)
 		insertPosition = nextHeadingLineNumber;
 	} else {
-		// No next heading - insert right after the heading line
 		insertPosition = headingLineNumber + 1;
 	}
 
-	// Insert the todo line
-	lines.splice(insertPosition, 0, todoLine);
+	if (addBlankLine) {
+		// Check if there's already a blank line at the insert position
+		const lineAfterHeading = lines[headingLineNumber + 1];
+		const alreadyHasBlank = lineAfterHeading !== undefined && lineAfterHeading.trim() === '';
+
+		if (alreadyHasBlank) {
+			// Insert after the existing blank line
+			lines.splice(headingLineNumber + 2, 0, todoLine);
+		} else {
+			// Insert blank line + todo
+			lines.splice(insertPosition, 0, '', todoLine);
+		}
+	} else {
+		lines.splice(insertPosition, 0, todoLine);
+	}
 
 	return lines.join('\n');
 }