Create run workspaces only when needed

internal/stage/cleanup.go

@@ -28,6 +28,8 @@ type cleanupSemanticConfig struct {
 
 func (cleanupStage) Name() string { return "cleanup" }
 
+func (cleanupStage) RequiresRunWorkspace() bool { return false }
+
 func (cleanupStage) Run(ctx context.Context, env *Env, m *manifest.Manifest) (*StageResult, error) {
 	plan, err := buildCleanupPlan(env, m)
 	if err != nil {

internal/stage/stage.go

@@ -43,6 +43,12 @@ type Stage interface {
 	Run(ctx context.Context, env *Env, m *manifest.Manifest) (*StageResult, error)
 }
 
+// RunWorkspacePolicy lets a stage declare whether it requires the current
+// invocation's run directory. Stages require it by default.
+type RunWorkspacePolicy interface {
+	RequiresRunWorkspace() bool
+}
+
 const maxResumeReasonLength = 512
 
 // ResumeValidation reports whether a previously succeeded stage can be reused.