Add GitWorktreeSandbox.cleanup_stale() and diff_against_head() class
methods, create strategy_actor module with resolve_strategy_actor(),
add PlanApplyService.correction_diff() method, and fix _get_apply_service()
and _get_plan_executor() to use correct API signatures.
Add Behave BDD unit tests and Robot Framework integration tests for all
new functionality.
ISSUES CLOSED: #8628