fix(repository): saturating_add for in-memory version bump

create_task used a plain += on the in-memory version returned to the
caller while FileSystemStorage uses saturating_add when serialising
the frontmatter. The two would disagree at u64::MAX, and in debug
builds the + operator would panic on overflow. Match the storage
behaviour.

crates/onyx-core/src/repository.rs

@@ -26,7 +26,10 @@ impl TaskRepository {
     // Task operations
     pub fn create_task(&mut self, list_id: Uuid, mut task: Task) -> Result<Task> {
         self.storage.write_task(list_id, &task)?;
-        task.version += 1;
+        // Mirror the saturating increment that FileSystemStorage applies to
+        // the on-disk frontmatter so the in-memory Task matches what was
+        // written and doesn't wrap at u64::MAX.
+        task.version = task.version.saturating_add(1);
         Ok(task)
     }