docs: correct the LFS trap and add Tools/check-lfs.sh

The lfs filter is inactive until 'git lfs install' writes filter.lfs.*, and a
matched file is then stored as a plain blob without any warning. Document that
instead of the wrong claim that git add fails, and add a guard script.

Docs/setup.md

@@ -47,10 +47,13 @@ Anything marked **human step** needs credentials, licence acceptance, or a decis
 ```bash
 git clone <remote> gig-line
 cd gig-line
-git lfs install          # requires git-lfs; otherwise every LFS-tracked add fails
+git lfs install          # writes filter.lfs.* into git config; required before any content commit
 git lfs pull
+Tools/check-lfs.sh       # fails when the lfs filter or the pointer storage is missing
 ```
 
+Without `git lfs install`, adding an LFS-matched file does not error — it commits the file as a plain blob. `Tools/check-lfs.sh` is the guard for that.
+
 Sizes: the engine lives outside the repository (pinned install path recorded in `toolchain.md`). The repository holds the Unreal project, Blender sources, geo sources, and tooling.
 
 ## Known blockers