Function update_task 

pub fn update_task(
    base: &Path,
    tasks: &HashMap<String, Task>,
    id_or_prefix: &str,
    status: Option<Status>,
    priority: Option<Priority>,
    tags: Option<Vec<String>>,
    assignee: Option<String>,
    body: Option<String>,
    title: Option<String>,
    parent: Option<Option<String>>,
) -> Result<Task>

Update task fields. Only Some fields are changed.

The parent parameter uses a double-Option to distinguish three states:

• None → leave parent unchanged
• Some(None) → clear parent (detach from any epic)
• Some(Some(id)) → set parent to the given epic ID (must exist)