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>

CLAUDE.md

@@ -95,3 +95,49 @@ From AGENTS.md and Obsidian best practices:
 - Keep startup lightweight; defer heavy work until needed
 - Use stable command IDs (don't rename after release)
 - Provide sensible defaults for all settings
+
+## Sidebar View Architecture
+
+### Key Files
+- `src/views/todo-sidebar-view.ts` — Main sidebar panel (`TodoSidebarView` extends `ItemView`)
+- `src/views/todo-item-component.ts` — Pure DOM factory for individual todo `<li>` elements
+- `src/core/todo-parser.ts` — Parses markdown into `TodoGroup[]` with nested `TodoItem` trees
+- `src/core/todo-transformer.ts` — Pure functions for modifying file content (toggle, move, indent)
+- `src/modals/note-select-modal.ts` — First step of cross-file move: pick target note
+- `src/modals/heading-select-modal.ts` — Second step: pick heading within target note
+
+### Data Flow
+1. `parseTodosGroupedByHeading(content)` → `TodoGroup[]` (each group has a heading + tree of todos)
+2. `buildTodoTree(flatTodos)` — stack-based algorithm: todos whose indent > parent's become children
+3. `renderGroups()` renders each group; `createTodoItemEl()` renders items recursively
+4. `flatTodoList: FlatTodoEntry[]` is rebuilt each render for keyboard navigation
+
+### Event Listener Lifecycle (Critical)
+- **Register keydown listener ONCE in `onOpen()`**, not in `renderGroups()` — otherwise listeners accumulate on every file change and arrow keys skip items
+- Use `this.currentFile` inside handlers instead of closure variables (file changes between renders)
+- `registerEvent()` wrappers handle cleanup automatically on view close
+
+### Keyboard Navigation
+- `focusedIndex = -1` means panel is not yet activated (no visual focus)
+- First ArrowDown/ArrowUp when `focusedIndex === -1` sets it to 0 (activates focus like Outline view)
+- `flatTodoList` flattens the entire tree depth-first; arrow keys walk this list linearly
+- Container has `tabindex="0"` so it can receive keyboard events without stealing focus on render
+
+### Drag and Drop
+- Parent `<li>` and all nested `<li>` are `draggable="true"`; `stopPropagation` on `dragstart` prevents child drag from bubbling to parent
+- `handleDragStart` stores `draggedTodo` and `draggedChildLines` (via `collectChildLineNumbers`)
+- Item drop zones use 3 zones (top/middle/bottom thirds):
+  - Top → insert above
+  - Middle → nest as child (increase indent, insert after target's last descendant)
+  - Bottom → insert below
+- Group (heading) drop zones allow dropping onto the heading area to move to end of that section
+- `performNest` adjusts indentation with `indentTodoLines(lines, delta)` before inserting
+
+### Hotkey Matching (Platform-Aware)
+- Uses `app.hotkeyManager.getHotkeys(commandId)` and `getDefaultHotkeys(commandId)` (internal API)
+- `Platform.isMacOS` distinguishes Mod (Cmd on Mac, Ctrl on Windows) from Ctrl
+- See `matchesHotkey()` in `todo-sidebar-view.ts` for the full matching logic
+
+### Cross-File Move
+- Always pass `addBlankLine: true` to `insertTodoUnderHeading` from modals — inserts a blank line between the heading and the moved todo for readability
+- In-file drag moves use `moveTodoWithChildren` directly (no blank line needed)