docs: sync markdown files with current codebase state

- Remove BottomSheet.svelte from PLAN.md file structure (deleted in
  efb4cca — NewTaskInput hand-rolls its own sheet)
- Expand workspace path validation description in API.md and CLAUDE.md
  to include filesystem root "/" alongside system directories, matching
  the forbidden list added in fix(tauri): reject "/" root path

https://claude.ai/code/session_015BSAnuhvMBLk7s4g7dSE53

CLAUDE.md

@@ -106,7 +106,7 @@ Pre-alpha. No users, no released builds, no data to migrate. Breaking changes to
 - Task deduplication on load (handles sync conflict duplicates)
 - Subtask hierarchy: subtask count shown on parent tasks in list, subtask detail via three-panel slide navigation, inline add at top of subtask list (new subtasks prepend), collapsible completed subtasks section, cascade delete (parent deletion removes all subtasks with confirmation warning)
 - Custom confirmation dialogs (ConfirmDialog component replaces native confirm())
-- Workspace path validation (rejects system directories)
+- Workspace path validation (rejects filesystem root `/` and system directories: `/etc`, `/usr`, `/bin`, `/sbin`, `/var`, `/proc`, `/sys`, `/dev`)
 - Task detail auto-cleanup (taskStack clears when viewed task is deleted or list switches)
 - Swipe gestures on mobile: swipe left/right on a task to toggle completion (swipe direction depends on current status)
 - Accessibility: ARIA labels/roles on interactive components, keyboard handlers, `prefers-reduced-motion` CSS support

PLAN.md

@@ -671,7 +671,6 @@ apps/tauri/
 │       │   ├── TaskItem.svelte
 │       │   ├── NewTaskInput.svelte
 │       │   ├── TaskDetailView.svelte
-│       │   ├── BottomSheet.svelte
 │       │   ├── ConfirmDialog.svelte
 │       │   └── DateTimePicker.svelte
 │       └── stores/

docs/API.md

@@ -454,7 +454,7 @@ All metadata and state files use an atomic write pattern (write to `.tmp` then r
 
 - **List names**: Rejected if they contain `/`, `\`, or `..` components. Canonicalized and verified to stay within workspace root.
 - **Sync paths**: Validated to reject `..` components and backslashes anywhere in the path before any file system operation.
-- **Workspace paths** (Tauri): Rejected if they point to system directories (`/etc`, `/usr`, `/bin`, etc.).
+- **Workspace paths** (Tauri): Rejected if they point to the filesystem root (`/`) or system directories (`/etc`, `/usr`, `/bin`, `/sbin`, `/var`, `/proc`, `/sys`, `/dev`).
 - **Filenames**: Sanitized to replace `/ \ : * ? " < > |` and control characters with `_`.
 
 ## Example: Complete Workflow