#2983 A nested structural swamp on a same-host remote worker waits on its own step's lock held by swamp serve

Pre-existing, found during the swamp-club#2955 plan review. The workflow execution service takes the per-step model lock (stepLockHook, src/domain/workflows/execution_service.ts around line 1764) before invokeMethod dispatches the step to a remote worker through resolvedPlacement. Serve therefore holds the step lock while the worker runs the step. If the worker runs on the same host and shares the datastore, a nested structural swamp spawned by the step on the worker is not a descendant of serve, so the drain in waitForPerModelLocks (src/cli/repo_context.ts) waits on the step's own lock until SWAMP_LOCK_TIMEOUT_MS and fails with LockTimeoutError.