docs: update stale documentation after audit fixes

Fix outdated thread safety note in API.md — Storage trait now requires
Send + Sync and Tauri uses Mutex<AppState>. Mark completed Phase 5
features in PLAN.md (subtasks, due dates, move tasks, keyboard shortcuts).
Fix CLAUDE.md formatting.

CLAUDE.md

@@ -32,6 +32,7 @@ Two-crate workspace (`resolver = "2"`, edition 2021) plus a Tauri app:
 - **onyx-core** — Pure Rust library. Storage trait with `FileSystemStorage` implementation, `TaskRepository` (main API), data models, config, error types. No CLI/UI dependencies. `keyring` feature-gated behind `keyring-storage` (default on) for Android compatibility.
 - **onyx-cli** — CLI frontend using clap. Commands are in `src/commands/` (init, workspace, list, task, group). Output formatting in `src/output.rs`.
 - **apps/tauri/** — Tauri v2 GUI. Svelte 5 frontend in `src/`, Rust backend in `src-tauri/` with Tauri commands that call into `onyx-core`. `notify` crate feature-gated for Android.
+
 ### Key patterns
 
 - **Storage trait** (`storage.rs`): Strategy pattern for task persistence. `FileSystemStorage` reads/writes markdown files with YAML frontmatter and JSON metadata files.

PLAN.md

@@ -856,17 +856,17 @@ npm run tauri ios build
 #### Desktop & Mobile
 - [x] Multiple task lists (folders)
 - [x] Switch between lists
-- [ ] Subtasks support
+- [x] Subtasks support
-- [ ] Due dates with date picker
+- [x] Due dates with date picker
 - [ ] Rich markdown editor for task notes
-- [ ] Move tasks between lists
+- [x] Move tasks between lists
 - [ ] Change storage folder location in settings
 - [ ] Search functionality
 - [x] Theme selection (light/dark mode)
 
 #### Desktop-Specific
 - [x] Drag & drop reordering
-- [ ] Keyboard shortcuts
+- [x] Keyboard shortcuts
 - [ ] Multiple windows (optional)
 
 #### Mobile-Specific

docs/API.md

@@ -441,10 +441,9 @@ Key test areas:
 
 ## Thread Safety
 
-**Note:** The current implementation is not thread-safe. If you need concurrent access:
+The `Storage` trait requires `Send + Sync`, and `TaskRepository` wraps `Box<dyn Storage + Send + Sync>`, so repository instances can be shared across threads behind a `Mutex`. The Tauri GUI uses `Mutex<AppState>` for this purpose.
 
-1. Use external synchronization (e.g., `Mutex<TaskRepository>`)
+For concurrent access:
-2. Create separate repository instances per thread (file system will handle locking)
-3. Consider implementing a service layer with proper locking
 
-Future versions may include built-in concurrency support.
+1. Wrap `TaskRepository` in `Mutex` or `RwLock` (the Tauri app does this)
+2. Or create separate repository instances per thread (file system handles locking)