- Define StepSessionExecutor interfaces and utility types (FN-1039)
- Implement StepSessionExecutor class with parallel wave execution, conflict detection, and retry logic (FN-1039)
- Add comprehensive test suite for StepSessionExecutor (1225 lines) (FN-1039)
- Wire step-session execution branch into TaskExecutor with pause, stuck-kill, and global pause handlers
- Track active step executors per task for lifecycle management and cleanup
- Export StepSessionExecutor and types from engine package index
- Add executor integration tests for step session lifecycle (388 lines)