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.

README.md

@@ -21,6 +21,7 @@ Pre-production. Workstation setup and repository scaffolding are in progress; no
 ```bash
 git lfs install    # required: content, images, audio, and geo files are LFS-tracked
 git lfs pull
+Tools/check-lfs.sh # verifies the lfs filter is active and files are stored as pointers
 ```
 
 The engine is installed outside the repository and pinned in [Docs/toolchain.md](Docs/toolchain.md). The full procedure is in [Docs/setup.md](Docs/setup.md).