Integrate previous-session artifact hydration into prepare stage

internal/app/runner.go

@@ -555,6 +555,12 @@ func needsObjectStoreForRun(cfg *config.Config, stages []stage.Stage) bool {
 	if cfg.Session.Inputs.AudioS3 != nil && stageRequested("prepare") {
 		return true
 	}
+	if stageRequested("prepare") {
+		requirements := artifacts.CollectPreviousArtifactRequirements(configuredScriptoriumArtifacts(cfg))
+		if len(requirements) > 0 && strings.TrimSpace(cfg.Session.PreviousSessionID) != "" {
+			return true
+		}
+	}
 	if !stageRequested("archive") {
 		return false
 	}
@@ -569,3 +575,10 @@ func needsObjectStoreForRun(cfg *config.Config, stages []stage.Stage) bool {
 	}
 	return true
 }
+
+func configuredScriptoriumArtifacts(cfg *config.Config) map[string]config.ScriptoriumArtifactConfig {
+	if cfg == nil || cfg.Pipeline == nil || cfg.Pipeline.Scriptorium == nil {
+		return nil
+	}
+	return cfg.Pipeline.Scriptorium.Artifacts
+}

internal/app/runner_test.go

@@ -199,6 +199,55 @@ func TestExecuteStagesAnalyzeOutputsPersistAsScriptoriumArtifacts(t *testing.T)
 	}
 }
 
+func TestNeedsObjectStoreForRunPrepareWithPreviousRequirements(t *testing.T) {
+	tests := []struct {
+		name              string
+		previousSessionID string
+		want              bool
+	}{
+		{
+			name:              "previous session configured",
+			previousSessionID: "2026-05-10",
+			want:              true,
+		},
+		{
+			name:              "previous session missing",
+			previousSessionID: "",
+			want:              false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			cfg := &config.Config{
+				Pipeline: &config.PipelineConfig{
+					Scriptorium: &config.ScriptoriumConfig{
+						Artifacts: map[string]config.ScriptoriumArtifactConfig{
+							"session_recap": {
+								Enabled: true,
+								Inputs: map[string]config.ScriptoriumInputConfig{
+									"previous_recap": {
+										Source:   "narratio.previous_session.artifact.session_recap",
+										Required: true,
+									},
+								},
+							},
+						},
+					},
+				},
+				Session: &config.SessionConfig{
+					PreviousSessionID: tt.previousSessionID,
+				},
+			}
+
+			got := needsObjectStoreForRun(cfg, []stage.Stage{countingStage{name: "prepare", runs: new(int)}})
+			if got != tt.want {
+				t.Fatalf("needsObjectStoreForRun() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
 func TestExecuteStagesArchiveFailsWhenRequiredRecapPromotionMissingForSelectedArtifacts(t *testing.T) {
 	cfg := testConfig(t)
 	cfg.Pipeline.Storage.S3 = &config.StorageS3Config{