Add remote session loading

internal/stage/prepare.go

@@ -113,6 +113,23 @@ func (prepareStage) Run(ctx context.Context, env *Env, m *manifest.Manifest) (*S
 	registerConfigInput := func(kind, path, checksum, source string) {
 		inputs = append(inputs, manifest.InputRecord{Kind: kind, Path: path, Checksum: checksum, Source: source})
 	}
+	registerSessionConfigInput := func(path, checksum string) {
+		source := env.Config.SessionSource
+		if strings.TrimSpace(source.Source) == "" {
+			source.Source = "session_config"
+		}
+		inputs = append(inputs, manifest.InputRecord{
+			Kind:      "session_config",
+			Path:      path,
+			Checksum:  checksum,
+			Source:    source.Source,
+			S3Bucket:  source.S3Bucket,
+			S3Key:     source.S3Key,
+			S3Size:    source.S3Size,
+			S3ETag:    source.S3ETag,
+			SpoolPath: source.SpoolPath,
+		})
+	}
 
 	campaignDst := filepath.Join(paths.InputsDir, "campaign.yml")
 	campaignChecksum, err := copyFileIfChanged(env.ArtifactStore, campaignSrc, campaignDst)
@@ -126,7 +143,7 @@ func (prepareStage) Run(ctx context.Context, env *Env, m *manifest.Manifest) (*S
 	if err != nil {
 		return nil, fmt.Errorf("prepare: materialize session.yml: %w", err)
 	}
-	registerConfigInput("session_config", sessionDst, sessionChecksum, "session_config")
+	registerSessionConfigInput(sessionDst, sessionChecksum)
 
 	pipelineResolvedBytes, err := renderResolvedPipeline(env.Config.Pipeline)
 	if err != nil {

internal/stage/prepare_test.go

@@ -152,6 +152,64 @@ func TestPrepareStageIdempotent(t *testing.T) {
 	}
 }
 
+func TestPrepareStageRecordsLocalSessionProvenance(t *testing.T) {
+	env, m := setupPrepareEnv(t)
+	root := filepath.Dir(env.Config.SessionPath)
+	writeFile(t, filepath.Join(root, "audio", "a.flac"), "a")
+	env.Config.Session.Inputs.AudioFiles = []string{"./audio/a.flac"}
+
+	if _, err := (prepareStage{}).Run(context.Background(), env, m); err != nil {
+		t.Fatalf("prepare.Run() error = %v", err)
+	}
+	sessionInput := findManifestInput(t, m.Inputs, "session_config")
+	if sessionInput.Source != "session_config" {
+		t.Fatalf("session source = %q, want session_config", sessionInput.Source)
+	}
+	if sessionInput.S3Bucket != "" || sessionInput.S3Key != "" || sessionInput.SpoolPath != "" {
+		t.Fatalf("local session input has unexpected remote provenance: %#v", sessionInput)
+	}
+}
+
+func TestPrepareStageRecordsRemoteSessionProvenance(t *testing.T) {
+	env, m := setupPrepareEnv(t)
+	remoteSessionPath := filepath.Join(t.TempDir(), "downloaded-session.yml")
+	writeFile(t, remoteSessionPath, "session_id: 2026-05-03\ninputs:\n  audio_s3:\n    prefix: audio/\n")
+	env.Config.SessionPath = remoteSessionPath
+	env.Config.Session.Inputs.AudioDir = ""
+	env.Config.Session.Inputs.AudioS3 = &config.SessionAudioS3Input{Prefix: "audio/"}
+	env.Config.SessionSource = config.SessionSource{
+		Source:    "session_config.s3",
+		LocalPath: remoteSessionPath,
+		S3Bucket:  "my-dnd-archive",
+		S3Key:     "dnd/campaigns/sample-campaign/sessions/2026-05-03/session.yml",
+		S3Size:    58,
+		S3ETag:    "session-etag",
+		SpoolPath: remoteSessionPath,
+	}
+	env.Config.Pipeline.Spool = config.SpoolConfig{Root: filepath.Join(t.TempDir(), "spool")}
+	env.Config.Pipeline.Storage.S3 = &config.StorageS3Config{Bucket: "my-dnd-archive", RootPrefix: "dnd"}
+	m.RunID = "20260515T031522Z-a1b2c3d4"
+	m.LocalWorkDir = artifacts.SessionRunRootForCampaign(env.Config.Pipeline.Workspace.Root, "sample-campaign", m.SessionID, m.RunID)
+	m.LocalSpoolDir = artifacts.SessionSpoolAudioDir(env.Config.Pipeline.Spool.Root, "sample-campaign", m.SessionID, m.RunID)
+	fake := &storage.FakeBackend{}
+	fake.SeedObject(storage.FakeObject{Key: "dnd/campaigns/sample-campaign/sessions/2026-05-03/audio/alice.flac", Data: []byte("alice")})
+	env.ObjectStore = fake
+
+	if _, err := (prepareStage{}).Run(context.Background(), env, m); err != nil {
+		t.Fatalf("prepare.Run() error = %v", err)
+	}
+	sessionInput := findManifestInput(t, m.Inputs, "session_config")
+	if sessionInput.Source != "session_config.s3" {
+		t.Fatalf("session source = %q, want session_config.s3", sessionInput.Source)
+	}
+	if sessionInput.S3Bucket != "my-dnd-archive" || sessionInput.S3Key != "dnd/campaigns/sample-campaign/sessions/2026-05-03/session.yml" {
+		t.Fatalf("remote session input missing bucket/key: %#v", sessionInput)
+	}
+	if sessionInput.S3Size != 58 || sessionInput.S3ETag != "session-etag" || sessionInput.SpoolPath != remoteSessionPath {
+		t.Fatalf("remote session input missing metadata: %#v", sessionInput)
+	}
+}
+
 func TestPrepareStageS3AudioDownloadAndMaterialization(t *testing.T) {
 	env, m := setupPrepareEnv(t)
 	env.Config.Session.Campaign = "forsaken"
@@ -543,3 +601,14 @@ func snapshotInputs(inputs []manifest.InputRecord) map[string]string {
 	}
 	return out
 }
+
+func findManifestInput(t *testing.T, inputs []manifest.InputRecord, kind string) manifest.InputRecord {
+	t.Helper()
+	for _, input := range inputs {
+		if input.Kind == kind {
+			return input
+		}
+	}
+	t.Fatalf("manifest input kind %q not found in %#v", kind, inputs)
+	return manifest.InputRecord{}
+}