Implement artifact-level render command with Markdown output and update docs

internal/cli/root.go

@@ -10,7 +10,7 @@ import (
 func NewRootCommand() *cobra.Command {
 	cmd := &cobra.Command{
 		Use:           "seriatim",
-		Short:         "Merge, trim, and normalize transcript artifacts",
+		Short:         "Merge, trim, normalize, and render transcript artifacts",
 		Version:       buildinfo.Version,
 		SilenceErrors: true,
 		SilenceUsage:  true,
@@ -18,6 +18,7 @@ func NewRootCommand() *cobra.Command {
 
 	cmd.AddCommand(newMergeCommand())
 	cmd.AddCommand(newNormalizeCommand())
+	cmd.AddCommand(newRenderCommand())
 	cmd.AddCommand(newTrimCommand())
 	return cmd
 }