chore: track *.vsix via Git LFS

Add .gitattributes routing *.vsix through Git LFS and convert the committed
extension vsix to an LFS pointer. Keeps the reproducible build artifact in the
repo without growing normal history on each rebuild. Future-only — existing
vsix blobs remain in history (a `git lfs migrate` rewrite would be needed to
purge those, deferred since origin/master is shared).

.gitattributes

@@ -0,0 +1 @@
+*.vsix filter=lfs diff=lfs merge=lfs -text