chore: add LFS tracking rules for screenshots and slide PNGs

Prerequisite commit before syncing binary assets. With these rules in
HEAD, subsequent git-add of PNG/PPTX files runs them through the
git-lfs clean filter (backed by git-xet transfer agent), producing
LFS pointer objects rather than raw blobs.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitattributes

@@ -50,3 +50,5 @@ saved_model/**/* filter=lfs diff=lfs merge=lfs -text
 *.zst filter=lfs diff=lfs merge=lfs -text
 *tfevents* filter=lfs diff=lfs merge=lfs -text
 *.pptx filter=lfs diff=lfs merge=lfs -text
+assets/screenshots/** filter=lfs diff=lfs merge=lfs -text
+slides/*.png filter=lfs diff=lfs merge=lfs -text