Add artifact provenance and stage skip outcomes

internal/stage/stage.go

@@ -44,8 +44,19 @@ type Stage interface {
 	Run(ctx context.Context, env *Env, m *manifest.Manifest) (*StageResult, error)
 }
 
+// StageDisposition describes the outcome of a stage that returned without an error.
+type StageDisposition string
+
+const (
+	// StageDispositionSucceeded is the zero value so existing stages remain successful.
+	StageDispositionSucceeded StageDisposition = ""
+	StageDispositionSkipped   StageDisposition = "skipped"
+)
+
 // StageResult is the declared output of a stage execution.
 type StageResult struct {
+	Disposition      StageDisposition
+	SkipReason       string
 	Outputs          []artifacts.Ref
 	Logs             []string
 	GeneratedConfigs []string