Fixed a path-duplication bug and added regression coverage

internal/artifacts/paths.go

@@ -4,6 +4,7 @@ import "path/filepath"
 
 // SessionPaths contains canonical local paths for one session work directory.
 type SessionPaths struct {
+	WorkspaceRoot            string
 	Root                     string
 	InputsDir                string
 	AudioDir                 string
@@ -26,6 +27,7 @@ func buildSessionPaths(workspaceRoot, sessionID string) SessionPaths {
 	root := SessionWorkDir(workspaceRoot, sessionID)
 	transcripts := filepath.Join(root, "transcripts")
 	return SessionPaths{
+		WorkspaceRoot:            workspaceRoot,
 		Root:                     root,
 		InputsDir:                filepath.Join(root, "inputs"),
 		AudioDir:                 filepath.Join(root, "audio"),