Extract a SubagentRunner fun-interface (SubagentRunRequest/SubagentRunResult)
and an InSessionSubagentRunner default that delegates to the existing
executeStage path, with per-run effectives captured in the lambda closure so
the seam stays free of the internal RunEffectives type.
DefaultSessionOrchestrator.enterStage now dispatches through subagentRunner.run
instead of calling executeStage inline; effectives threading is dropped from
run/step/executeMove/enterStage/resume. ReplayOrchestrator supplies its own
runner. Behaviour is identical — RefinementLoopTest and full check stay green.
