Initialize narratio Go module and CLI scaffold

internal/adapters/seriatim/runner.go

@@ -0,0 +1,20 @@
+// Package seriatim declares the adapter contract for transcript merge execution.
+package seriatim
+
+import "context"
+
+// Runner is a placeholder adapter interface for seriatim invocation.
+type Runner interface {
+	Run(ctx context.Context, req MergeRequest) (MergeResult, error)
+}
+
+// MergeRequest is a placeholder merge input.
+type MergeRequest struct {
+	TranscriptPaths []string
+	OutputPath      string
+}
+
+// MergeResult is a placeholder merge output.
+type MergeResult struct {
+	MergedPath string
+}