singleFileMerge: Merging all file / directory selected in one file.

**Describe the solution you'd like**

I would like to merge all `src/generated/**/*.ts` into one `src/generated/index.ts`

Start by locating the singleFileMerge entry point and its existing tests. Trace how selected src/generated/**/*.ts files are read and determine the project’s expected ordering and merge behavior. Done means those files can be combined into src/generated/index.ts without changing the intended generated output.

Written by the indexing model from the issue text.