Detach prepared execution ownership from runners

internal/usecase/prepared_execution.go

@@ -22,7 +22,7 @@ const (
 
 // PreparedExecution owns one frozen, single-use runner execution.
 type PreparedExecution struct {
-	owner   *Runner
+	owner   *runnerIdentity
 	mu      sync.Mutex
 	state   preparedExecutionState
 	details *domain.PreparedRun
@@ -63,7 +63,7 @@ func (r *Runner) PrepareExecution(ctx context.Context, req domain.RunRequest) (*
 	}
 
 	return &PreparedExecution{
-		owner:   r,
+		owner:   r.identity,
 		state:   preparedExecutionReady,
 		details: details,
 		payload: &preparedExecutionPayload{
@@ -170,7 +170,7 @@ func (r *Runner) RunPrepared(ctx context.Context, prepared *PreparedExecution) (
 }
 
 func (p *PreparedExecution) claim(owner *Runner) (*preparedExecutionPayload, error) {
-	if p == nil || owner == nil || p.owner != owner {
+	if p == nil || owner == nil || owner.identity == nil || p.owner != owner.identity {
 		return nil, fmt.Errorf("%w: prepared execution does not belong to this runner", ErrInvalidRequest)
 	}