Function is_task_runnable 

pub fn is_task_runnable(
    task: &Task,
    active: &QueueFile,
    done: Option<&QueueFile>,
) -> bool

Check if a task is runnable (dependencies met and scheduling satisfied).

A task is runnable if:

• All dependencies are met (depends_on tasks are Done or Rejected)
• The scheduled_start time has passed (or is not set)