docs: mark subtask hierarchy as done in PLAN.md and CLAUDE.md

https://claude.ai/code/session_01XWcSekaCAJ7qnr4hN3tgL8

CLAUDE.md

@@ -87,11 +87,11 @@ The GUI uses Svelte 5 runes mode (`$state`, `$derived`, `$effect`, `$props()`).
 - Desktop packaging (Linux: AppImage + .deb)
 - Flutter GUI at full parity with Tauri (WebDAV UI, has_time, sync status, sync mode)
 - Tauri desktop-only deps (notify, keyring) feature-gated for Android compilation
+- Subtask hierarchy (expand/collapse in task list, inline add in detail view, cascade toggle/delete)
 
 ### GUI features NOT yet done
 
 - Workspace retarget/migrate
-- Subtask hierarchy (data model exists, not used anywhere)
 - Search/filter tasks
 - Desktop packaging for Windows and macOS
 

PLAN.md

@@ -724,7 +724,7 @@ WorkspaceConfig {
 - [x] Sync status indicators (last-sync time + upload/download counts chip in TasksScreen)
 - [x] Push/pull sync mode selection (session-only sync direction selector in SettingsScreen)
 - [x] Group-by-due-date toggle per list (checkmark toggle in list kebab menu)
-- [ ] Subtask hierarchy (data model exists, needs UI)
+- [x] Subtask hierarchy (expand/collapse, inline add, cascade toggle/delete)
 - [ ] Search/filter tasks
 - [x] Desktop packaging (Linux: AppImage + .deb; Windows/macOS not yet verified)
 - [x] File watcher (notify crate, 500ms debounce, auto-reloads UI on external file changes)