#363 CRITICAL: sleeping_tasks.discard() called on dict - AttributeError

## Issue In engine/durable_context.py:1389, the code calls discard() on sleeping_tasks which is a dict: ```python # Line 383: sleeping_tasks is a dict self.sleeping_tasks: dict[str, str] = {} # Line 1389: discard() is a SET method, not dict method! self.sleeping_tasks.discard(task_id) # CRASH: AttributeError ``` Line 1429 correctly uses pop(): self.sleeping_tasks.pop(task_id, None) ## Impact - Runtime crash when marking sleeping tasks as executed - Workflow execution fails unexpectedly ## Location engine/durable_context.py:1389 ## Fix Change to: self.sleeping_tasks.pop(task_id, None)