feat(FN-1040): add StepSessionExecutor for parallel step execution and integrate into executor

- 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)

packages/engine/src/index.ts

@@ -19,6 +19,8 @@ export { StuckTaskDetector, type StuckTaskDetectorOptions, type DisposableSessio
 export { TokenCapDetector, type TokenCapCheckResult } from "./token-cap-detector.js";
 export { SelfHealingManager, type SelfHealingOptions } from "./self-healing.js";
 export { ProjectManager } from "./project-manager.js";
+export { StepSessionExecutor } from "./step-session-executor.js";
+export type { StepResult, ParallelWave, StepSessionExecutorOptions } from "./step-session-executor.js";
 // Multi-project runtime types
 export {
   type ProjectRuntime,