Ln 400 Story Executor

This is a runtime-backed orchestrator that takes a Story with planned tasks and executes them in strict priority order: tasks in To Review first, then To Rework, then Todo. It manages git worktrees, checkpoints progress so you can resume, and forces an immediate ln-402 review after every implementation or rework step. The runtime prevents retry storms by comparing artifacts between loops and pausing if nothing changed. It handles parallel task groups when explicitly marked but otherwise runs tasks sequentially, moving the Story to To Review when done but never to Done. You'd reach for this when you have a fully planned Story ready to implement and want deterministic, resumable execution with quality gates built in.