Complete diagnostics retention semantics

internal/core/diagnostics/run_dir.go

@@ -21,6 +21,31 @@ type RunDirectory struct {
 	createdAt time.Time
 }
 
+type RetentionDecisionInput struct {
+	RetentionMode         string
+	RunSucceeded          bool
+	HasSkippedCorrections bool
+}
+
+func ShouldRetainRunDirectory(input RetentionDecisionInput) bool {
+	// Failed runs are always retained.
+	if !input.RunSucceeded {
+		return true
+	}
+
+	switch input.RetentionMode {
+	case "always":
+		return true
+	case "never":
+		return true
+	case "auto":
+		return input.HasSkippedCorrections
+	default:
+		// Be conservative for unknown values.
+		return true
+	}
+}
+
 // InvocationMetadata captures non-secret invocation details for diagnostics.
 type InvocationMetadata struct {
 	Operation      string    `json:"operation"`
@@ -187,20 +212,14 @@ func (r *RunDirectory) WriteChunkingSummary(summary *chunking.DetailedSummary) e
 	return nil
 }
 
-// TODO: In later phases, implement full "auto" semantics based on skip/report outcomes.
-func (r *RunDirectory) ApplyRetention(runSucceeded bool) error {
-	if !runSucceeded {
-		// Failed runs are always retained.
-		return nil
+func (r *RunDirectory) ApplyRetention(input RetentionDecisionInput) error {
+	decision := input
+	if decision.RetentionMode == "" {
+		decision.RetentionMode = r.retention
 	}
 
-	switch r.retention {
-	case "always":
-		return nil
-	case "never", "auto":
-		return os.RemoveAll(r.path)
-	default:
-		// Retain by default for unknown modes; config validation should prevent this.
+	if ShouldRetainRunDirectory(decision) {
 		return nil
 	}
+	return os.RemoveAll(r.path)
 }