fix(todo-sync): provide default priority to prevent SQLite NOT NULL violation

extractPriority() returns undefined when task metadata has no priority
field, but OpenCode's TodoTable requires priority as NOT NULL. This
causes a silent SQLiteError that prevents all Task→Todo syncing.

Add ?? "medium" fallback so todos always have a valid priority.

src/tools/task/todo-sync.ts

@@ -65,7 +65,7 @@ export function syncTaskToTodo(task: Task): TodoInfo | null {
     id: task.id,
     content: task.subject,
     status: todoStatus,
-    priority: extractPriority(task.metadata),
+    priority: extractPriority(task.metadata) ?? "medium",
   };
 }