fix(storage): atomic task-file writes

write_task used plain fs::write for the .md payload even though every
other write path in this module (metadata files, sync state, offline
queue, config) uses atomic_write. A crash mid-write left a truncated
.md file whose malformed YAML frontmatter then failed list_tasks for
the entire list. Route through atomic_write so a failed write either
leaves the old file intact or produces the full new file.

crates/onyx-core/src/storage.rs

@@ -381,7 +381,9 @@ impl Storage for FileSystemStorage {
         }
 
         let content = self.write_markdown_with_frontmatter(task)?;
-        fs::write(&task_path, content)?;
+        // Atomic write: a crash mid-write must not leave a truncated .md file
+        // that then fails YAML parsing on the next list_tasks/read_task.
+        atomic_write(&task_path, content.as_bytes())?;
 
         // Update list metadata to include this task in task_order if not already present
         let mut list_metadata = self.read_list_metadata(list_id)?;